diff --git a/.github/pull_request_template.md b/.github/pull_request_template.md
index ff8e182b..4ff5bcc3 100644
--- a/.github/pull_request_template.md
+++ b/.github/pull_request_template.md
@@ -7,7 +7,6 @@
 <!-- These checkboxes serve for the maintainers of PQClean to verify your submission. Please do not check them yourself. -->
 
 * [ ] `#if`/`#ifdef`s only for header encapsulation
-* [ ] `api.h` does not include other files
 * [ ] No stringification macros
 * [ ] Output-parameter pointers in functions are on the left
 * [ ] Negative return values on failure of API functions (within restrictions of FO transform).
diff --git a/README.md b/README.md
index e69225ed..f68dbdd8 100644
--- a/README.md
+++ b/README.md
@@ -34,6 +34,7 @@ _The checking of items on this list is still being developed. Checked items shou
 * [x] Code is valid C99
 * [x] Passes functional tests
 * [x] API functions do not write outside provided buffers
+* [x] `api.h` cannot include external files
 * [x] Compiles with `-Wall -Wextra -Wpedantic -Werror` with `gcc` and `clang`
 * [x] Consistent test vectors across runs
 * [x] Consistent test vectors on big-endian and little-endian machines
diff --git a/test/test_api_h.py b/test/test_api_h.py
new file mode 100644
index 00000000..501791f9
--- /dev/null
+++ b/test/test_api_h.py
@@ -0,0 +1,32 @@
+import os
+import re
+
+import pqclean
+
+
+def test_preprocessor():
+    for scheme in pqclean.Scheme.all_schemes():
+        for implementation in scheme.implementations:
+            yield check_preprocessor, implementation
+
+
+def check_preprocessor(implementation: pqclean.Implementation):
+    apipath = os.path.join(implementation.path(), 'api.h')
+    errors = []
+    p = re.compile(r'^\s*#include\s*"')
+    with open(apipath) as f:
+        for i, line in enumerate(f):
+            if p.match(line):
+                errors.append("\n at {}:{}".format(apipath, i+1))
+    if errors:
+        raise AssertionError(
+            "Prohibited external include in api.h" + "".join(errors)
+        )
+
+if __name__ == "__main__":
+    try:
+        import nose2
+        nose2.main()
+    except ImportError:
+        import nose
+        nose.runmodule()