diff --git a/README.md b/README.md
index f6d51b2fa3137..683dae80c194f 100644
--- a/README.md
+++ b/README.md
@@ -1237,8 +1237,8 @@ For more, see [flake8-pytest-style](https://pypi.org/project/flake8-pytest-style
 | PT013 | incorrect-pytest-import | Found incorrect import of pytest, use simple `import pytest` instead |  |
 | PT015 | assert-always-false | Assertion always fails, replace with `pytest.fail()` |  |
 | PT016 | fail-without-message | No message passed to `pytest.fail()` |  |
-| PT017 | assert-in-except | Found assertion on exception `{name}` in except block, use `pytest.raises()` instead |  |
-| PT018 | composite-assertion | Assertion should be broken down into multiple parts |  |
+| PT017 | assert-in-except | Found assertion on exception `{name}` in `except` block, use `pytest.raises()` instead |  |
+| PT018 | [composite-assertion](https://beta.ruff.rs/docs/rules/composite-assertion/) | Assertion should be broken down into multiple parts | 🛠 |
 | PT019 | fixture-param-without-value | Fixture `{name}` without value is injected as parameter, use `@pytest.mark.usefixtures` instead |  |
 | PT020 | deprecated-yield-fixture | `@pytest.yield_fixture` is deprecated, use `@pytest.fixture` |  |
 | PT021 | fixture-finalizer-callback | Use `yield` instead of `request.addfinalizer` |  |
diff --git a/crates/ruff/resources/test/fixtures/flake8_pytest_style/PT018.py b/crates/ruff/resources/test/fixtures/flake8_pytest_style/PT018.py
index 4fc893abcb214..e94c5d1ec79be 100644
--- a/crates/ruff/resources/test/fixtures/flake8_pytest_style/PT018.py
+++ b/crates/ruff/resources/test/fixtures/flake8_pytest_style/PT018.py
@@ -6,13 +6,24 @@ def test_ok():
     assert something or something_else
     assert something or something_else and something_third
     assert not (something and something_else)
-
+    assert something, "something message"
+    assert something or something_else and something_third, "another message"
 
 def test_error():
     assert something and something_else
     assert something and something_else and something_third
     assert something and not something_else
     assert something and (something_else or something_third)
+    assert not something and something_else
     assert not (something or something_else)
     assert not (something or something_else or something_third)
+
+    # recursive case
+    assert not (a or not (b or c))
+    assert not (a or not (b and c))  # note that we only reduce once here
+
+    # detected, but no autofix for messages
+    assert something and something_else, "error message"
+    assert not (something or something_else and something_third), "with message"
+    # detected, but no autofix for mixed conditions (e.g. `a or b and c`)
     assert not (something or something_else and something_third)
diff --git a/crates/ruff/src/checkers/ast.rs b/crates/ruff/src/checkers/ast.rs
index ef5534fc02220..ec8d0a46c438a 100644
--- a/crates/ruff/src/checkers/ast.rs
+++ b/crates/ruff/src/checkers/ast.rs
@@ -1572,11 +1572,12 @@ where
                     }
                 }
                 if self.settings.rules.enabled(&Rule::CompositeAssertion) {
-                    if let Some(diagnostic) =
-                        flake8_pytest_style::rules::composite_condition(stmt, test)
-                    {
-                        self.diagnostics.push(diagnostic);
-                    }
+                    flake8_pytest_style::rules::composite_condition(
+                        self,
+                        stmt,
+                        test,
+                        msg.as_deref(),
+                    );
                 }
             }
             StmtKind::With { items, body, .. } => {
diff --git a/crates/ruff/src/rules/flake8_pytest_style/rules/assertion.rs b/crates/ruff/src/rules/flake8_pytest_style/rules/assertion.rs
index b7aa172b98f5f..54ad0d3f735bb 100644
--- a/crates/ruff/src/rules/flake8_pytest_style/rules/assertion.rs
+++ b/crates/ruff/src/rules/flake8_pytest_style/rules/assertion.rs
@@ -1,27 +1,69 @@
-use ruff_macros::{define_violation, derive_message_formats};
 use rustpython_parser::ast::{
     Boolop, Excepthandler, ExcepthandlerKind, Expr, ExprKind, Keyword, Stmt, StmtKind, Unaryop,
 };
 
-use super::helpers::is_falsy_constant;
-use super::unittest_assert::UnittestAssert;
-use crate::ast::helpers::unparse_stmt;
+use ruff_macros::{define_violation, derive_message_formats};
+
+use crate::ast::helpers::{create_expr, create_stmt, unparse_stmt};
 use crate::ast::types::Range;
 use crate::ast::visitor;
 use crate::ast::visitor::Visitor;
 use crate::checkers::ast::Checker;
 use crate::fix::Fix;
 use crate::registry::Diagnostic;
-use crate::violation::{AlwaysAutofixableViolation, Violation};
+use crate::source_code::Stylist;
+use crate::violation::{AlwaysAutofixableViolation, AutofixKind, Availability, Violation};
+
+use super::helpers::is_falsy_constant;
+use super::unittest_assert::UnittestAssert;
 
 define_violation!(
-    pub struct CompositeAssertion;
+    /// ## What it does
+    /// Checks for assertions that combine multiple independent conditions.
+    ///
+    /// ## Why is this bad?
+    /// Composite assertion statements are harder debug upon failure, as the
+    /// failure message will not indicate which condition failed.
+    ///
+    /// ## Example
+    /// ```python
+    /// def test_foo():
+    ///     assert something and something_else
+    ///
+    /// def test_bar():
+    ///     assert not (something or something_else)
+    /// ```
+    ///
+    /// Use instead:
+    /// ```python
+    /// def test_foo():
+    ///     assert something
+    ///     assert something_else
+    ///
+    /// def test_bar():
+    ///     assert not something
+    ///     assert not something_else
+    /// ```
+    pub struct CompositeAssertion {
+        pub fixable: bool,
+    }
 );
 impl Violation for CompositeAssertion {
+    const AUTOFIX: Option<AutofixKind> = Some(AutofixKind::new(Availability::Sometimes));
+
     #[derive_message_formats]
     fn message(&self) -> String {
         format!("Assertion should be broken down into multiple parts")
     }
+
+    fn autofix_title_formatter(&self) -> Option<fn(&Self) -> String> {
+        let CompositeAssertion { fixable } = self;
+        if *fixable {
+            Some(|_| format!("Break down assertion into multiple parts"))
+        } else {
+            None
+        }
+    }
 }
 
 define_violation!(
@@ -34,7 +76,7 @@ impl Violation for AssertInExcept {
     fn message(&self) -> String {
         let AssertInExcept { name } = self;
         format!(
-            "Found assertion on exception `{name}` in except block, use `pytest.raises()` instead"
+            "Found assertion on exception `{name}` in `except` block, use `pytest.raises()` instead"
         )
     }
 }
@@ -119,22 +161,6 @@ where
     }
 }
 
-/// Check if the test expression is a composite condition.
-/// For example, `a and b` or `not (a or b)`. The latter is equivalent
-/// to `not a and not b` by De Morgan's laws.
-const fn is_composite_condition(test: &Expr) -> bool {
-    match &test.node {
-        ExprKind::BoolOp {
-            op: Boolop::And, ..
-        } => true,
-        ExprKind::UnaryOp {
-            op: Unaryop::Not,
-            operand,
-        } => matches!(&operand.node, ExprKind::BoolOp { op: Boolop::Or, .. }),
-        _ => false,
-    }
-}
-
 fn check_assert_in_except(name: &str, body: &[Stmt]) -> Vec<Diagnostic> {
     // Walk body to find assert statements that reference the exception name
     let mut visitor = ExceptionHandlerVisitor::new(name);
@@ -180,11 +206,11 @@ pub fn unittest_assertion(
 }
 
 /// PT015
-pub fn assert_falsy(assert_stmt: &Stmt, test_expr: &Expr) -> Option<Diagnostic> {
-    if is_falsy_constant(test_expr) {
+pub fn assert_falsy(stmt: &Stmt, test: &Expr) -> Option<Diagnostic> {
+    if is_falsy_constant(test) {
         Some(Diagnostic::new(
             AssertAlwaysFalse,
-            Range::from_located(assert_stmt),
+            Range::from_located(stmt),
         ))
     } else {
         None
@@ -207,14 +233,129 @@ pub fn assert_in_exception_handler(handlers: &[Excepthandler]) -> Vec<Diagnostic
         .collect()
 }
 
+/// Check if the test expression is a composite condition.
+/// For example, `a and b` or `not (a or b)`. The latter is equivalent
+/// to `not a and not b` by De Morgan's laws.
+const fn is_composite_condition(test: &Expr) -> bool {
+    match &test.node {
+        ExprKind::BoolOp {
+            op: Boolop::And, ..
+        } => true,
+        ExprKind::UnaryOp {
+            op: Unaryop::Not,
+            operand,
+        } => matches!(&operand.node, ExprKind::BoolOp { op: Boolop::Or, .. }),
+        _ => false,
+    }
+}
+
+/// Negate condition, i.e. `a` => `not a` and `not a` => `a`
+fn negate(f: Expr) -> Expr {
+    match f.node {
+        ExprKind::UnaryOp {
+            op: Unaryop::Not,
+            operand,
+        } => *operand,
+        _ => create_expr(ExprKind::UnaryOp {
+            op: Unaryop::Not,
+            operand: Box::new(f),
+        }),
+    }
+}
+
+/// Return `true` if the condition appears to be a fixable composite condition.
+fn is_fixable_composite_condition(test: &Expr) -> bool {
+    let ExprKind::UnaryOp {
+        op: Unaryop::Not,
+        operand,
+    } = &test.node else {
+         return true;
+    };
+    let ExprKind::BoolOp {
+        op: Boolop::Or,
+        values,
+    } = &operand.node else {
+         return true;
+     };
+    // Only take cases without mixed `and` and `or`
+    values.iter().all(|expr| {
+        !matches!(
+            expr.node,
+            ExprKind::BoolOp {
+                op: Boolop::And,
+                ..
+            }
+        )
+    })
+}
+
+/// Replace composite condition `assert a == "hello" and b == "world"` with two statements
+/// `assert a == "hello"` and `assert b == "world"`.
+///
+/// It's assumed that the condition is fixable, i.e., that `is_fixable_composite_condition`
+/// returns `true`.
+fn fix_composite_condition(stylist: &Stylist, stmt: &Stmt, test: &Expr) -> Fix {
+    let mut conditions: Vec<Expr> = vec![];
+    match &test.node {
+        ExprKind::BoolOp {
+            op: Boolop::And,
+            values,
+        } => {
+            // Compound, so split (Split)
+            conditions.extend(values.clone());
+        }
+        ExprKind::UnaryOp {
+            op: Unaryop::Not,
+            operand,
+        } => {
+            match &operand.node {
+                ExprKind::BoolOp {
+                    op: Boolop::Or,
+                    values,
+                } => {
+                    // `not (a or b)` equals `not a and not b`
+                    let vals = values
+                        .iter()
+                        .map(|f| negate(f.clone()))
+                        .collect::<Vec<Expr>>();
+
+                    // Compound, so split (Split)
+                    conditions.extend(vals);
+                }
+                _ => {
+                    // Do not split
+                    conditions.push(*operand.clone());
+                }
+            }
+        }
+        _ => {}
+    };
+
+    // for each condition create `assert condition`
+    let mut content: Vec<String> = Vec::with_capacity(conditions.len());
+    for condition in conditions {
+        content.push(unparse_stmt(
+            &create_stmt(StmtKind::Assert {
+                test: Box::new(condition.clone()),
+                msg: None,
+            }),
+            stylist,
+        ));
+    }
+
+    let content = content.join(stylist.line_ending().as_str());
+    Fix::replacement(content, stmt.location, stmt.end_location.unwrap())
+}
+
 /// PT018
-pub fn composite_condition(assert_stmt: &Stmt, test_expr: &Expr) -> Option<Diagnostic> {
-    if is_composite_condition(test_expr) {
-        Some(Diagnostic::new(
-            CompositeAssertion,
-            Range::from_located(assert_stmt),
-        ))
-    } else {
-        None
+pub fn composite_condition(checker: &mut Checker, stmt: &Stmt, test: &Expr, msg: Option<&Expr>) {
+    if is_composite_condition(test) {
+        let fixable = msg.is_none() && is_fixable_composite_condition(test);
+        let mut diagnostic =
+            Diagnostic::new(CompositeAssertion { fixable }, Range::from_located(stmt));
+        if fixable && checker.patch(diagnostic.kind.rule()) {
+            diagnostic.amend(fix_composite_condition(checker.stylist, stmt, test));
+        }
+        checker.diagnostics.push(diagnostic);
     }
 }
diff --git a/crates/ruff/src/rules/flake8_pytest_style/snapshots/ruff__rules__flake8_pytest_style__tests__PT018.snap b/crates/ruff/src/rules/flake8_pytest_style/snapshots/ruff__rules__flake8_pytest_style__tests__PT018.snap
index fa5ba420680ad..0a627c28873b2 100644
--- a/crates/ruff/src/rules/flake8_pytest_style/snapshots/ruff__rules__flake8_pytest_style__tests__PT018.snap
+++ b/crates/ruff/src/rules/flake8_pytest_style/snapshots/ruff__rules__flake8_pytest_style__tests__PT018.snap
@@ -1,74 +1,219 @@
 ---
-source: src/rules/flake8_pytest_style/mod.rs
+source: crates/ruff/src/rules/flake8_pytest_style/mod.rs
 expression: diagnostics
 ---
 - kind:
-    CompositeAssertion: ~
-  location:
-    row: 12
-    column: 4
-  end_location:
-    row: 12
-    column: 39
-  fix: ~
-  parent: ~
-- kind:
-    CompositeAssertion: ~
+    CompositeAssertion:
+      fixable: true
   location:
     row: 13
     column: 4
   end_location:
     row: 13
-    column: 59
-  fix: ~
+    column: 39
+  fix:
+    content:
+      - assert something
+      - assert something_else
+    location:
+      row: 13
+      column: 4
+    end_location:
+      row: 13
+      column: 39
   parent: ~
 - kind:
-    CompositeAssertion: ~
+    CompositeAssertion:
+      fixable: true
   location:
     row: 14
     column: 4
   end_location:
     row: 14
-    column: 43
-  fix: ~
+    column: 59
+  fix:
+    content:
+      - assert something
+      - assert something_else
+      - assert something_third
+    location:
+      row: 14
+      column: 4
+    end_location:
+      row: 14
+      column: 59
   parent: ~
 - kind:
-    CompositeAssertion: ~
+    CompositeAssertion:
+      fixable: true
   location:
     row: 15
     column: 4
   end_location:
     row: 15
-    column: 60
-  fix: ~
+    column: 43
+  fix:
+    content:
+      - assert something
+      - assert not something_else
+    location:
+      row: 15
+      column: 4
+    end_location:
+      row: 15
+      column: 43
   parent: ~
 - kind:
-    CompositeAssertion: ~
+    CompositeAssertion:
+      fixable: true
   location:
     row: 16
     column: 4
   end_location:
     row: 16
-    column: 44
-  fix: ~
+    column: 60
+  fix:
+    content:
+      - assert something
+      - assert something_else or something_third
+    location:
+      row: 16
+      column: 4
+    end_location:
+      row: 16
+      column: 60
   parent: ~
 - kind:
-    CompositeAssertion: ~
+    CompositeAssertion:
+      fixable: true
   location:
     row: 17
     column: 4
   end_location:
     row: 17
-    column: 63
-  fix: ~
+    column: 43
+  fix:
+    content:
+      - assert not something
+      - assert something_else
+    location:
+      row: 17
+      column: 4
+    end_location:
+      row: 17
+      column: 43
   parent: ~
 - kind:
-    CompositeAssertion: ~
+    CompositeAssertion:
+      fixable: true
   location:
     row: 18
     column: 4
   end_location:
     row: 18
+    column: 44
+  fix:
+    content:
+      - assert not something
+      - assert not something_else
+    location:
+      row: 18
+      column: 4
+    end_location:
+      row: 18
+      column: 44
+  parent: ~
+- kind:
+    CompositeAssertion:
+      fixable: true
+  location:
+    row: 19
+    column: 4
+  end_location:
+    row: 19
+    column: 63
+  fix:
+    content:
+      - assert not something
+      - assert not something_else
+      - assert not something_third
+    location:
+      row: 19
+      column: 4
+    end_location:
+      row: 19
+      column: 63
+  parent: ~
+- kind:
+    CompositeAssertion:
+      fixable: true
+  location:
+    row: 22
+    column: 4
+  end_location:
+    row: 22
+    column: 34
+  fix:
+    content:
+      - assert not a
+      - assert b or c
+    location:
+      row: 22
+      column: 4
+    end_location:
+      row: 22
+      column: 34
+  parent: ~
+- kind:
+    CompositeAssertion:
+      fixable: true
+  location:
+    row: 23
+    column: 4
+  end_location:
+    row: 23
+    column: 35
+  fix:
+    content:
+      - assert not a
+      - assert b and c
+    location:
+      row: 23
+      column: 4
+    end_location:
+      row: 23
+      column: 35
+  parent: ~
+- kind:
+    CompositeAssertion:
+      fixable: false
+  location:
+    row: 26
+    column: 4
+  end_location:
+    row: 26
+    column: 56
+  fix: ~
+  parent: ~
+- kind:
+    CompositeAssertion:
+      fixable: false
+  location:
+    row: 27
+    column: 4
+  end_location:
+    row: 27
+    column: 80
+  fix: ~
+  parent: ~
+- kind:
+    CompositeAssertion:
+      fixable: false
+  location:
+    row: 29
+    column: 4
+  end_location:
+    row: 29
     column: 64
   fix: ~
   parent: ~