diff --git a/changelog/10819.deprecation.rst b/changelog/10819.deprecation.rst
new file mode 100644
index 00000000000..ebb306379b7
--- /dev/null
+++ b/changelog/10819.deprecation.rst
@@ -0,0 +1,3 @@
+Added a deprecation warning for class-scoped fixtures defined as instance methods (without ``@classmethod``). Such fixtures set attributes on a different instance than the test methods use, leading to unexpected behavior. Use ``@classmethod`` decorator instead -- by :user:`yastcher`.
+
+See :issue:`10819` and :issue:`14011`.
diff --git a/doc/en/deprecations.rst b/doc/en/deprecations.rst
index 73b04f03711..d6fca825792 100644
--- a/doc/en/deprecations.rst
+++ b/doc/en/deprecations.rst
@@ -149,6 +149,53 @@ You can fix it by converting generators and iterators to lists or tuples:
 Note that :class:`range` objects are ``Collection`` and are not affected by this deprecation.
 
 
+.. _class-scoped-fixture-as-instance-method:
+
+Class-scoped fixture as instance method
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. deprecated:: 9.1
+
+Defining a class-scoped fixture as an instance method (without ``@classmethod``) is deprecated
+and will be removed in pytest 10.0.
+
+When a class-scoped fixture is defined as an instance method, any attributes set on ``self``
+will not be visible to test methods. This happens because pytest creates a new instance of the
+test class for each test method, while the fixture runs only once per class on a different instance.
+
+**Before** (deprecated):
+
+.. code-block:: python
+
+    class TestExample:
+        @pytest.fixture(scope="class")
+        def setup_data(self):
+            self.data = [1, 2, 3]  # This won't be visible to tests!
+
+        def test_something(self, setup_data):
+            assert self.data == [
+                1,
+                2,
+                3,
+            ]  # AttributeError: 'TestExample' object has no attribute 'data'
+
+**After** (recommended):
+
+.. code-block:: python
+
+    class TestExample:
+        @pytest.fixture(scope="class")
+        @classmethod
+        def setup_data(cls):
+            cls.data = [1, 2, 3]
+
+        def test_something(self, setup_data):
+            assert self.data == [1, 2, 3]  # Works correctly
+
+Using ``@classmethod`` ensures attributes are set on the class itself, making them accessible
+to all test methods.
+
+
 .. _monkeypatch-fixup-namespace-packages:
 
 ``monkeypatch.syspath_prepend`` with legacy namespace packages
diff --git a/src/_pytest/deprecated.py b/src/_pytest/deprecated.py
index f25db4df287..d9da941a789 100644
--- a/src/_pytest/deprecated.py
+++ b/src/_pytest/deprecated.py
@@ -34,6 +34,14 @@
     "Use @pytest.fixture instead; they are the same."
 )
 
+CLASS_FIXTURE_INSTANCE_METHOD = PytestRemovedIn10Warning(
+    "Class-scoped fixture defined as instance method is deprecated.\n"
+    "Instance attributes set in this fixture will NOT be visible to test methods,\n"
+    "as each test gets a new instance while the fixture runs only once per class.\n"
+    "Use @classmethod decorator and set attributes on cls instead.\n"
+    "See https://docs.pytest.org/en/stable/deprecations.html#class-scoped-fixture-as-instance-method"
+)
+
 # This deprecation is never really meant to be removed.
 PRIVATE = PytestDeprecationWarning("A private pytest class or function was used.")
 
diff --git a/src/_pytest/fixtures.py b/src/_pytest/fixtures.py
index 367eb6419de..da29622ef23 100644
--- a/src/_pytest/fixtures.py
+++ b/src/_pytest/fixtures.py
@@ -55,6 +55,7 @@
 from _pytest.config import ExitCode
 from _pytest.config.argparsing import Parser
 from _pytest.deprecated import check_ispytest
+from _pytest.deprecated import CLASS_FIXTURE_INSTANCE_METHOD
 from _pytest.deprecated import FIXTURE_GETFIXTUREVALUE_DURING_TEARDOWN
 from _pytest.deprecated import YIELD_FIXTURE
 from _pytest.main import Session
@@ -1220,6 +1221,16 @@ def resolve_fixture_function(
     # request.instance so that code working with "fixturedef" behaves
     # as expected.
     instance = request.instance
+
+    if fixturedef._scope is Scope.Class:
+        # Check if fixture is an instance method (bound to instance, not class)
+        if hasattr(fixturefunc, "__self__"):
+            bound_to = fixturefunc.__self__
+            # classmethod: bound_to is the class itself (a type)
+            # instance method: bound_to is an instance (not a type)
+            if not isinstance(bound_to, type):
+                warnings.warn(CLASS_FIXTURE_INSTANCE_METHOD, stacklevel=2)
+
     if instance is not None:
         # Handle the case where fixture is defined not in a test class, but some other class
         # (for example a plugin class with a fixture), see #2270.
diff --git a/testing/deprecated_test.py b/testing/deprecated_test.py
index c49a6e084ce..dd5c6781869 100644
--- a/testing/deprecated_test.py
+++ b/testing/deprecated_test.py
@@ -85,3 +85,24 @@ def __init__(self, foo: int, *, _ispytest: bool = False) -> None:
 
     # Doesn't warn.
     PrivateInit(10, _ispytest=True)
+
+
+def test_class_scope_instance_method_is_deprecated(pytester: Pytester) -> None:
+    pytester.makepyfile(
+        """
+        import pytest
+
+        class TestClass:
+            @pytest.fixture(scope="class")
+            def fix(self):
+                self.attr = True
+
+            def test_foo(self, fix):
+                pass
+        """
+    )
+    result = pytester.runpytest("-Werror::pytest.PytestRemovedIn10Warning")
+    result.assert_outcomes(errors=1)
+    result.stdout.fnmatch_lines(
+        ["*PytestRemovedIn10Warning: Class-scoped fixture defined as instance method*"]
+    )