diff --git a/changelog/TBD.deprecation.rst b/changelog/TBD.deprecation.rst
new file mode 100644
index 00000000000..ed9779e1f50
--- /dev/null
+++ b/changelog/TBD.deprecation.rst
@@ -0,0 +1,6 @@
+The ``pytest._fillfuncargs`` function is now deprecated. This function was kept
+for backward compatibility with an older plugin.
+
+It's functionality is not meant to be used directly, but if you must replace
+it, use `function._request._fillfixtures()` instead, though note this is not
+a public API and may break in the future.
diff --git a/doc/en/deprecations.rst b/doc/en/deprecations.rst
index 13d59bce2b7..4d8177a543d 100644
--- a/doc/en/deprecations.rst
+++ b/doc/en/deprecations.rst
@@ -20,6 +20,19 @@ Below is a complete list of all pytest features which are considered deprecated.
 :ref:`standard warning filters <warnings>`.
 
 
+The ``pytest._fillfuncargs`` function
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. deprecated:: 5.5
+
+This function was kept for backward compatibility with an older plugin.
+
+It's functionality is not meant to be used directly, but if you must replace
+it, use `function._request._fillfixtures()` instead, though note this is not
+a public API and may break in the future.
+
+
+
 ``--no-print-logs`` command-line option
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/src/_pytest/deprecated.py b/src/_pytest/deprecated.py
index 926cfcf19dc..2295bfe1d28 100644
--- a/src/_pytest/deprecated.py
+++ b/src/_pytest/deprecated.py
@@ -25,6 +25,11 @@
     "since pytest 2.3 - use the newer attribute instead."
 )
 
+FILLFUNCARGS = PytestDeprecationWarning(
+    "The `_fillfuncargs` function is deprecated, use "
+    "function._request._fillfixtures() instead if you must."
+)
+
 RESULT_LOG = PytestDeprecationWarning(
     "--result-log is deprecated, please try the new pytest-reportlog plugin.\n"
     "See https://docs.pytest.org/en/latest/deprecations.html#result-log-result-log for more information."
diff --git a/src/_pytest/fixtures.py b/src/_pytest/fixtures.py
index 0200b803c75..1813020f650 100644
--- a/src/_pytest/fixtures.py
+++ b/src/_pytest/fixtures.py
@@ -28,6 +28,7 @@
 from _pytest.compat import NOTSET
 from _pytest.compat import safe_getattr
 from _pytest.compat import TYPE_CHECKING
+from _pytest.deprecated import FILLFUNCARGS
 from _pytest.deprecated import FIXTURE_POSITIONAL_ARGUMENTS
 from _pytest.deprecated import FUNCARGNAMES
 from _pytest.mark import ParameterSet
@@ -276,6 +277,7 @@ def reorder_items_atscope(items, argkeys_cache, items_by_argkey, scopenum):
 
 def fillfixtures(function):
     """ fill missing funcargs for a test function. """
+    warnings.warn(FILLFUNCARGS, stacklevel=2)
     function._request._fillfixtures()
 
 
diff --git a/testing/deprecated_test.py b/testing/deprecated_test.py
index cf7dee854aa..caa44854f5f 100644
--- a/testing/deprecated_test.py
+++ b/testing/deprecated_test.py
@@ -1,4 +1,5 @@
 import inspect
+from unittest import mock
 
 import pytest
 from _pytest import deprecated
@@ -146,3 +147,11 @@ def test_foo():
     )
 
     assert_no_print_logs(testdir, ())
+
+
+def test__fillfuncargs_is_deprecated() -> None:
+    with pytest.warns(
+        pytest.PytestDeprecationWarning,
+        match="The `_fillfuncargs` function is deprecated",
+    ):
+        pytest._fillfuncargs(mock.Mock())