diff --git a/docs/changelog/3433.bugfix.rst b/docs/changelog/3433.bugfix.rst
new file mode 100644
index 0000000000..47009b71b6
--- /dev/null
+++ b/docs/changelog/3433.bugfix.rst
@@ -0,0 +1,2 @@
+Normalize extra names when resolving dependencies so that underscores and hyphens are treated equivalently (e.g.
+``extras = snake_case`` now matches ``Provides-Extra: snake-case`` in wheel metadata) - by :user:`Fridayai700`.
diff --git a/src/tox/tox_env/python/virtual_env/package/util.py b/src/tox/tox_env/python/virtual_env/package/util.py
index a35ca43d5f..4ffd9dfe57 100644
--- a/src/tox/tox_env/python/virtual_env/package/util.py
+++ b/src/tox/tox_env/python/virtual_env/package/util.py
@@ -34,23 +34,24 @@ def dependencies_with_extras_from_markers(
     *,
     available_extras: set[str] | None = None,
 ) -> list[Requirement]:
-    if available_extras is not None and extras:
+    normalized_extras = {canonicalize_name(e) for e in extras}
+    if available_extras is not None and normalized_extras:
         normalized_available = {canonicalize_name(e) for e in available_extras}
-        if unknown := extras - normalized_available:
+        if unknown := normalized_extras - normalized_available:
             available_str = ", ".join(sorted(normalized_available)) or "none"
             unknown_str = ", ".join(sorted(unknown))
             msg = f"extras not found for package {package_name}: {unknown_str} (available: {available_str})"
             raise Fail(msg)
     result: list[Requirement] = []
     found: set[str] = set()
-    todo: set[str | None] = extras | {None}
+    todo: set[str | None] = normalized_extras | {None}
     visited: set[str | None] = set()
     while todo:
         new_extras: set[str | None] = set()
         for req, extra_markers in deps_with_markers:
             if todo & extra_markers:
                 if req.name == package_name:  # support for recursive extras
-                    new_extras.update(req.extras or set())
+                    new_extras.update(canonicalize_name(e) for e in (req.extras or set()))
                 else:
                     req_str = str(req)
                     if req_str not in found:
@@ -81,7 +82,7 @@ def _extract_extra_markers(req: Requirement) -> tuple[Requirement, set[str | Non
     while marker:
         extra = _get_extra(marker)
         if extra is not None:
-            extra_markers.add(extra)
+            extra_markers.add(canonicalize_name(extra))
             if new_markers and new_markers[-1] in {"and", "or"}:
                 del new_markers[-1]
             marker = markers.pop(0) if markers else None
diff --git a/tests/tox_env/python/virtual_env/package/test_python_package_util.py b/tests/tox_env/python/virtual_env/package/test_python_package_util.py
index c5cd000eb4..4c834cea5e 100644
--- a/tests/tox_env/python/virtual_env/package/test_python_package_util.py
+++ b/tests/tox_env/python/virtual_env/package/test_python_package_util.py
@@ -112,3 +112,34 @@ def test_validate_extras_normalization() -> None:
 def test_validate_extras_no_available() -> None:
     with pytest.raises(Fail, match=r"extras not found for package pkg: alpha \(available: none\)"):
         dependencies_with_extras([], {"alpha"}, "pkg", available_extras=set())
+
+
+def test_extras_underscore_hyphen_matching() -> None:
+    """Extras with underscores in tox.ini should match hyphens in metadata markers (#3433)."""
+    requires = [
+        Requirement('dep-a; extra == "kebab-case"'),
+        Requirement('dep-b; extra == "snake-case"'),
+        Requirement('dep-c; extra == "kebab-case-2"'),
+    ]
+    result = dependencies_with_extras(requires, {"kebab-case", "snake_case", "kebab-case-2"}, "pkg")
+    assert sorted(str(r) for r in result) == ["dep-a", "dep-b", "dep-c"]
+
+
+def test_extras_underscore_in_markers() -> None:
+    """Extras with underscores in markers should match hyphens in tox.ini (#3433)."""
+    requires = [
+        Requirement('dep-a; extra == "snake_case"'),
+    ]
+    result = dependencies_with_extras(requires, {"snake-case"}, "pkg")
+    assert [str(r) for r in result] == ["dep-a"]
+
+
+def test_extras_normalization_with_recursive() -> None:
+    """Recursive extras with underscores should be resolved correctly (#3433)."""
+    requires = [
+        Requirement('dep1; extra == "my-extra"'),
+        Requirement('name[sub_extra]; extra == "my-extra"'),
+        Requirement('dep2; extra == "sub-extra"'),
+    ]
+    result = dependencies_with_extras(requires, {"my_extra"}, "name")
+    assert sorted(str(r) for r in result) == ["dep1", "dep2"]