diff --git a/Tests/helper.py b/Tests/helper.py
index b333c2fd4ef..88c1f02a803 100644
--- a/Tests/helper.py
+++ b/Tests/helper.py
@@ -158,7 +158,7 @@ def assert_tuple_approx_equal(actuals, targets, threshold, msg):
     assert value, msg + ": " + repr(actuals) + " != " + repr(targets)
 
 
-def skip_unless_feature(feature):
+def skip_unless_feature(feature: str) -> pytest.MarkDecorator:
     reason = f"{feature} not available"
     return pytest.mark.skipif(not features.check(feature), reason=reason)
 
diff --git a/Tests/oss-fuzz/fuzz_font.py b/Tests/oss-fuzz/fuzz_font.py
index bc2ba9a7e27..8788d7021d3 100755
--- a/Tests/oss-fuzz/fuzz_font.py
+++ b/Tests/oss-fuzz/fuzz_font.py
@@ -23,7 +23,7 @@
     import fuzzers
 
 
-def TestOneInput(data):
+def TestOneInput(data: bytes) -> None:
     try:
         fuzzers.fuzz_font(data)
     except Exception:
@@ -32,7 +32,7 @@ def TestOneInput(data):
         pass
 
 
-def main():
+def main() -> None:
     fuzzers.enable_decompressionbomb_error()
     atheris.Setup(sys.argv, TestOneInput)
     atheris.Fuzz()
diff --git a/Tests/oss-fuzz/fuzz_pillow.py b/Tests/oss-fuzz/fuzz_pillow.py
index 545daccb680..e6e99d415a6 100644
--- a/Tests/oss-fuzz/fuzz_pillow.py
+++ b/Tests/oss-fuzz/fuzz_pillow.py
@@ -23,7 +23,7 @@
     import fuzzers
 
 
-def TestOneInput(data):
+def TestOneInput(data: bytes) -> None:
     try:
         fuzzers.fuzz_image(data)
     except Exception:
@@ -32,7 +32,7 @@ def TestOneInput(data):
         pass
 
 
-def main():
+def main() -> None:
     fuzzers.enable_decompressionbomb_error()
     atheris.Setup(sys.argv, TestOneInput)
     atheris.Fuzz()
diff --git a/Tests/oss-fuzz/fuzzers.py b/Tests/oss-fuzz/fuzzers.py
index 3f3c1e38833..3afa952157b 100644
--- a/Tests/oss-fuzz/fuzzers.py
+++ b/Tests/oss-fuzz/fuzzers.py
@@ -5,18 +5,18 @@
 from PIL import Image, ImageDraw, ImageFile, ImageFilter, ImageFont
 
 
-def enable_decompressionbomb_error():
+def enable_decompressionbomb_error() -> None:
     ImageFile.LOAD_TRUNCATED_IMAGES = True
     warnings.filterwarnings("ignore")
     warnings.simplefilter("error", Image.DecompressionBombWarning)
 
 
-def disable_decompressionbomb_error():
+def disable_decompressionbomb_error() -> None:
     ImageFile.LOAD_TRUNCATED_IMAGES = False
     warnings.resetwarnings()
 
 
-def fuzz_image(data):
+def fuzz_image(data: bytes) -> None:
     # This will fail on some images in the corpus, as we have many
     # invalid images in the test suite.
     with Image.open(io.BytesIO(data)) as im:
@@ -25,7 +25,7 @@ def fuzz_image(data):
         im.save(io.BytesIO(), "BMP")
 
 
-def fuzz_font(data):
+def fuzz_font(data: bytes) -> None:
     wrapper = io.BytesIO(data)
     try:
         font = ImageFont.truetype(wrapper)
diff --git a/Tests/oss-fuzz/test_fuzzers.py b/Tests/oss-fuzz/test_fuzzers.py
index 68834045a52..028ee71eec8 100644
--- a/Tests/oss-fuzz/test_fuzzers.py
+++ b/Tests/oss-fuzz/test_fuzzers.py
@@ -23,7 +23,7 @@
     "path",
     subprocess.check_output("find Tests/images -type f", shell=True).split(b"\n"),
 )
-def test_fuzz_images(path):
+def test_fuzz_images(path: str) -> None:
     fuzzers.enable_decompressionbomb_error()
     try:
         with open(path, "rb") as f:
@@ -54,7 +54,7 @@ def test_fuzz_images(path):
 @pytest.mark.parametrize(
     "path", subprocess.check_output("find Tests/fonts -type f", shell=True).split(b"\n")
 )
-def test_fuzz_fonts(path):
+def test_fuzz_fonts(path: str) -> None:
     if not path:
         return
     with open(path, "rb") as f: