diff --git a/src/poetry/core/packages/dependency.py b/src/poetry/core/packages/dependency.py
index a539d59b6..244867714 100644
--- a/src/poetry/core/packages/dependency.py
+++ b/src/poetry/core/packages/dependency.py
@@ -179,6 +179,8 @@ def marker(self, marker: Union[str, "BaseMarker"]) -> None:
                 for _, extra in or_:
                     self.in_extras.append(extra)
 
+        # Recalculate python versions.
+        self._python_versions = "*"
         if "python_version" in markers:
             ors = []
             for or_ in markers["python_version"]:
@@ -213,7 +215,8 @@ def marker(self, marker: Union[str, "BaseMarker"]) -> None:
                 ors.append(" ".join(ands))
 
             self._python_versions = " || ".join(ors)
-            self._python_constraint = parse_constraint(self._python_versions)
+
+        self._python_constraint = parse_constraint(self._python_versions)
 
     @property
     def transitive_marker(self) -> "BaseMarker":
diff --git a/tests/packages/test_dependency.py b/tests/packages/test_dependency.py
index 6038417f9..2be3e8b4d 100644
--- a/tests/packages/test_dependency.py
+++ b/tests/packages/test_dependency.py
@@ -305,3 +305,13 @@ def test_dependency_markers_are_the_same_as_markers():
     marker = parse_marker('extra=="bar"')
 
     assert dependency.marker == marker
+
+
+def test_marker_properly_unsets_python_constraint():
+    dependency = Dependency("foo", "^1.2.3")
+
+    dependency.marker = 'python_version >= "3.6"'
+    assert str(dependency.python_constraint) == ">=3.6"
+
+    dependency.marker = "*"
+    assert str(dependency.python_constraint) == "*"