diff --git a/mypy/typeshed/stubs/librt/librt/base64.pyi b/mypy/typeshed/stubs/librt/librt/base64.pyi
index 36366f5754ce..1cea838505d6 100644
--- a/mypy/typeshed/stubs/librt/librt/base64.pyi
+++ b/mypy/typeshed/stubs/librt/librt/base64.pyi
@@ -1 +1,2 @@
 def b64encode(s: bytes) -> bytes: ...
+def b64decode(s: bytes | str) -> bytes: ...
diff --git a/mypyc/lib-rt/librt_base64.c b/mypyc/lib-rt/librt_base64.c
index 020a56e412f4..1720359ef9a6 100644
--- a/mypyc/lib-rt/librt_base64.c
+++ b/mypyc/lib-rt/librt_base64.c
@@ -1,11 +1,16 @@
 #define PY_SSIZE_T_CLEAN
 #include <Python.h>
+#include <stdbool.h>
 #include "librt_base64.h"
 #include "libbase64.h"
 #include "pythoncapi_compat.h"
 
 #ifdef MYPYC_EXPERIMENTAL
 
+static PyObject *
+b64decode_handle_invalid_input(
+    PyObject *out_bytes, char *outbuf, size_t max_out, const char *src, size_t srclen);
+
 #define BASE64_MAXBIN ((PY_SSIZE_T_MAX - 3) / 2)
 
 #define STACK_BUFFER_SIZE 1024
@@ -63,11 +68,193 @@ b64encode(PyObject *self, PyObject *const *args, size_t nargs) {
     return b64encode_internal(args[0]);
 }
 
+static inline int
+is_valid_base64_char(char c, bool allow_padding) {
+    return ((c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') ||
+            (c >= '0' && c <= '9') || (c == '+') || (c == '/') || (allow_padding && c == '='));
+}
+
+static PyObject *
+b64decode_internal(PyObject *arg) {
+    const char *src;
+    Py_ssize_t srclen_ssz;
+
+    // Get input pointer and length
+    if (PyBytes_Check(arg)) {
+        src = PyBytes_AS_STRING(arg);
+        srclen_ssz = PyBytes_GET_SIZE(arg);
+    } else if (PyUnicode_Check(arg)) {
+        if (!PyUnicode_IS_ASCII(arg)) {
+            PyErr_SetString(PyExc_ValueError,
+                            "string argument should contain only ASCII characters");
+            return NULL;
+        }
+        src = (const char *)PyUnicode_1BYTE_DATA(arg);
+        srclen_ssz = PyUnicode_GET_LENGTH(arg);
+    } else {
+        PyErr_SetString(PyExc_TypeError,
+                        "argument should be a bytes-like object or ASCII string");
+        return NULL;
+    }
+
+    // Fast-path: empty input
+    if (srclen_ssz == 0) {
+        return PyBytes_FromStringAndSize(NULL, 0);
+    }
+
+    // Quickly ignore invalid characters at the end. Other invalid characters
+    // are also accepted, but they need a slow path.
+    while (srclen_ssz > 0 && !is_valid_base64_char(src[srclen_ssz - 1], true)) {
+        srclen_ssz--;
+    }
+
+    // Compute an output capacity that's at least 3/4 of input, without overflow:
+    // ceil(3/4 * N) == N - floor(N/4)
+    size_t srclen = (size_t)srclen_ssz;
+    size_t max_out = srclen - (srclen / 4);
+    if (max_out == 0) {
+        max_out = 1; // defensive (srclen > 0 implies >= 1 anyway)
+    }
+    if (max_out > (size_t)PY_SSIZE_T_MAX) {
+        PyErr_SetString(PyExc_OverflowError, "input too large");
+        return NULL;
+    }
+
+    // Allocate output bytes (uninitialized) of the max capacity
+    PyObject *out_bytes = PyBytes_FromStringAndSize(NULL, (Py_ssize_t)max_out);
+    if (out_bytes == NULL) {
+        return NULL; // Propagate memory error
+    }
+
+    char *outbuf = PyBytes_AS_STRING(out_bytes);
+    size_t outlen = max_out;
+
+    int ret = base64_decode(src, srclen, outbuf, &outlen, 0);
+
+    if (ret != 1) {
+        if (ret == 0) {
+            // Slow path: handle non-base64 input
+            return b64decode_handle_invalid_input(out_bytes, outbuf, max_out, src, srclen);
+        }
+        Py_DECREF(out_bytes);
+        if (ret == -1) {
+            PyErr_SetString(PyExc_NotImplementedError, "base64 codec not available in this build");
+        } else {
+            PyErr_SetString(PyExc_RuntimeError, "base64_decode failed");
+        }
+        return NULL;
+    }
+
+    // Sanity-check contract (decoder must not overflow our buffer)
+    if (outlen > max_out) {
+        Py_DECREF(out_bytes);
+        PyErr_SetString(PyExc_RuntimeError, "decoder wrote past output buffer");
+        return NULL;
+    }
+
+    // Shrink in place to the actual decoded length
+    if (_PyBytes_Resize(&out_bytes, (Py_ssize_t)outlen) < 0) {
+        // _PyBytes_Resize sets an exception and may free the old object
+        return NULL;
+    }
+    return out_bytes;
+}
+
+// Process non-base64 input by ignoring non-base64 characters, for compatibility
+// with stdlib b64decode.
+static PyObject *
+b64decode_handle_invalid_input(
+    PyObject *out_bytes, char *outbuf, size_t max_out, const char *src, size_t srclen)
+{
+    // Copy input to a temporary buffer, with non-base64 characters and extra suffix
+    // characters removed
+    size_t newbuf_len = 0;
+    char *newbuf = PyMem_Malloc(srclen);
+    if (newbuf == NULL) {
+        Py_DECREF(out_bytes);
+        return PyErr_NoMemory();
+    }
+
+    // Copy base64 characters and some padding to the new buffer
+    for (size_t i = 0; i < srclen; i++) {
+        char c = src[i];
+        if (is_valid_base64_char(c, false)) {
+            newbuf[newbuf_len++] = c;
+        } else if (c == '=') {
+            // Copy a necessary amount of padding
+            int remainder = newbuf_len % 4;
+            if (remainder == 0) {
+                // No padding needed
+                break;
+            }
+            int numpad = 4 - remainder;
+            // Check that there is at least the required amount padding (CPython ignores
+            // extra padding)
+            while (numpad > 0) {
+                if (i == srclen || src[i] != '=') {
+                    break;
+                }
+                newbuf[newbuf_len++] = '=';
+                i++;
+                numpad--;
+                // Skip non-base64 alphabet characters within padding
+                while (i < srclen && !is_valid_base64_char(src[i], true)) {
+                    i++;
+                }
+            }
+            break;
+        }
+    }
+
+    // Stdlib always performs a non-strict padding check
+    if (newbuf_len % 4 != 0) {
+        Py_DECREF(out_bytes);
+        PyMem_Free(newbuf);
+        PyErr_SetString(PyExc_ValueError, "Incorrect padding");
+        return NULL;
+    }
+
+    size_t outlen = max_out;
+    int ret = base64_decode(newbuf, newbuf_len, outbuf, &outlen, 0);
+    PyMem_Free(newbuf);
+
+    if (ret != 1) {
+        Py_DECREF(out_bytes);
+        if (ret == 0) {
+            PyErr_SetString(PyExc_ValueError, "Only base64 data is allowed");
+        }
+        if (ret == -1) {
+            PyErr_SetString(PyExc_NotImplementedError, "base64 codec not available in this build");
+        } else {
+            PyErr_SetString(PyExc_RuntimeError, "base64_decode failed");
+        }
+        return NULL;
+    }
+
+    // Shrink in place to the actual decoded length
+    if (_PyBytes_Resize(&out_bytes, (Py_ssize_t)outlen) < 0) {
+        // _PyBytes_Resize sets an exception and may free the old object
+        return NULL;
+    }
+    return out_bytes;
+}
+
+
+static PyObject*
+b64decode(PyObject *self, PyObject *const *args, size_t nargs) {
+    if (nargs != 1) {
+        PyErr_SetString(PyExc_TypeError, "b64decode() takes exactly one argument");
+        return 0;
+    }
+    return b64decode_internal(args[0]);
+}
+
 #endif
 
 static PyMethodDef librt_base64_module_methods[] = {
 #ifdef MYPYC_EXPERIMENTAL
-    {"b64encode", (PyCFunction)b64encode, METH_FASTCALL, PyDoc_STR("Encode bytes-like object using Base64.")},
+    {"b64encode", (PyCFunction)b64encode, METH_FASTCALL, PyDoc_STR("Encode bytes object using Base64.")},
+    {"b64decode", (PyCFunction)b64decode, METH_FASTCALL, PyDoc_STR("Decode a Base64 encoded bytes object or ASCII string.")},
 #endif
     {NULL, NULL, 0, NULL}
 };
@@ -111,7 +298,7 @@ static PyModuleDef_Slot librt_base64_module_slots[] = {
 static PyModuleDef librt_base64_module = {
     .m_base = PyModuleDef_HEAD_INIT,
     .m_name = "base64",
-    .m_doc = "base64 encoding and decoding optimized for mypyc",
+    .m_doc = "Fast base64 encoding and decoding optimized for mypyc",
     .m_size = 0,
     .m_methods = librt_base64_module_methods,
     .m_slots = librt_base64_module_slots,
diff --git a/mypyc/test-data/run-base64.test b/mypyc/test-data/run-base64.test
index 0f9151c2b00b..8d7eb7c13482 100644
--- a/mypyc/test-data/run-base64.test
+++ b/mypyc/test-data/run-base64.test
@@ -1,8 +1,9 @@
 [case testAllBase64Features_librt_experimental]
 from typing import Any
 import base64
+import binascii
 
-from librt.base64 import b64encode
+from librt.base64 import b64encode, b64decode
 
 from testutil import assertRaises
 
@@ -44,6 +45,111 @@ def test_encode_wrapper() -> None:
     with assertRaises(TypeError):
         enc(b"x", b"y")
 
+def test_decode_basic() -> None:
+    assert b64decode(b"eA==") == b"x"
+
+    with assertRaises(TypeError):
+        b64decode(bytearray(b"eA=="))
+
+    for non_ascii in "\x80", "foo\u100bar", "foo\ua1234bar":
+        with assertRaises(ValueError):
+            b64decode(non_ascii)
+
+def check_decode(b: bytes, encoded: bool = False) -> None:
+     if encoded:
+         enc = b
+     else:
+         enc = b64encode(b)
+     assert b64decode(enc) == getattr(base64, "b64decode")(enc)
+     if getattr(enc, "isascii")():  # Test stub has no "isascii"
+        enc_str = enc.decode("ascii")
+        assert b64decode(enc_str) == getattr(base64, "b64decode")(enc_str)
+
+def test_decode_different_strings() -> None:
+    for i in range(256):
+        check_decode(bytes([i]))
+        check_decode(bytes([i]) + b"x")
+        check_decode(bytes([i]) + b"xy")
+        check_decode(bytes([i]) + b"xyz")
+        check_decode(bytes([i]) + b"xyza")
+        check_decode(b"x" + bytes([i]))
+        check_decode(b"xy" + bytes([i]))
+        check_decode(b"xyz" + bytes([i]))
+        check_decode(b"xyza" + bytes([i]))
+
+    b = b"a\x00\xb7" * 1000
+    for i in range(1000):
+        check_decode(b[:i])
+
+    for b in b"", b"ab", b"bac", b"1234", b"xyz88", b"abc" * 200:
+        check_decode(b)
+
+def is_base64_char(x: int) -> bool:
+    c = chr(x)
+    return ('a' <= c <= 'z') or ('A' <= c <= 'Z') or ('0' <= c <= '9') or c in '+/='
+
+def test_decode_with_non_base64_chars() -> None:
+    # For stdlib compatibility, non-base64 characters should be ignored.
+
+    # Invalid characters as a suffix use a fast path.
+    check_decode(b"eA== ", encoded=True)
+    check_decode(b"eA==\n", encoded=True)
+    check_decode(b"eA== \t\n", encoded=True)
+    check_decode(b"\n", encoded=True)
+
+    check_decode(b" e  A = = ", encoded=True)
+
+    # Special case: Two different encodings of the same data
+    check_decode(b"eAa=", encoded=True)
+    check_decode(b"eAY=", encoded=True)
+
+    for x in range(256):
+        if not is_base64_char(x):
+            b = bytes([x])
+            check_decode(b, encoded=True)
+            check_decode(b"eA==" + b, encoded=True)
+            check_decode(b"e" + b + b"A==", encoded=True)
+            check_decode(b"eA=" + b + b"=", encoded=True)
+
+def check_decode_error(b: bytes, ignore_stdlib: bool = False) -> None:
+    if not ignore_stdlib:
+        with assertRaises(binascii.Error):
+            getattr(base64, "b64decode")(b)
+
+    # The raised error is different, since librt shouldn't depend on binascii
+    with assertRaises(ValueError):
+        b64decode(b)
+
+def test_decode_with_invalid_padding() -> None:
+    check_decode_error(b"eA")
+    check_decode_error(b"eA=")
+    check_decode_error(b"eHk")
+    check_decode_error(b"eA = ")
+
+    # Here stdlib behavior seems nonsensical, so we don't try to duplicate it
+    check_decode_error(b"eA=a=", ignore_stdlib=True)
+
+def test_decode_with_extra_data_after_padding() -> None:
+    check_decode(b"=", encoded=True)
+    check_decode(b"==", encoded=True)
+    check_decode(b"===", encoded=True)
+    check_decode(b"====", encoded=True)
+    check_decode(b"eA===", encoded=True)
+    check_decode(b"eHk==", encoded=True)
+    check_decode(b"eA==x", encoded=True)
+    check_decode(b"eHk=x", encoded=True)
+    check_decode(b"eA==abc=======efg", encoded=True)
+
+def test_decode_wrapper() -> None:
+    dec: Any = b64decode
+    assert dec(b"eA==") == b"x"
+
+    with assertRaises(TypeError):
+        dec()
+
+    with assertRaises(TypeError):
+        dec(b"x", b"y")
+
 [case testBase64FeaturesNotAvailableInNonExperimentalBuild_librt_base64]
 # This also ensures librt.base64 can be built without experimental features
 import librt.base64