@@ -26,6 +26,7 @@
     "samp",
     "stats",
     "table",
+    "tests",
     "time",
     "timeseries",
     "uncertainty",
 
@@ -3,7 +3,11 @@
 import pkgutil
 import subprocess
 import sys
+from pathlib import Path
 from textwrap import dedent
+from types import ModuleType
+
+import astropy
 
 
 def test_imports():
@@ -46,3 +50,20 @@ def test_toplevel_lazy_imports():
     assert 'astropy.units' in sys.modules
     """)
     assert subprocess.check_call([sys.executable, "-c", cmd]) == 0
+
+
+def test_completeness_toplevel__all__():
+    # Implicitly check that all items in __all__ exist.
+    all_items = {getattr(astropy, attr) for attr in astropy.__all__}
+    # Verify that the list of modules in __all__ is complete.
+    module_names = {
+        item.__name__.partition(".")[2]
+        for item in all_items
+        if isinstance(item, ModuleType)
+    }
+    module_dirs = {
+        f.name
+        for f in Path(astropy.__file__).parent.glob("[a-z]*")
+        if f.is_dir() and f.name != "extern"
+    }
+    assert module_names == module_dirs