diff --git a/ci/gpu/build.sh b/ci/gpu/build.sh
index 53ad948b61c..4acdc372817 100755
--- a/ci/gpu/build.sh
+++ b/ci/gpu/build.sh
@@ -1,5 +1,5 @@
 #!/bin/bash
-# Copyright (c) 2018-2021, NVIDIA CORPORATION.
+# Copyright (c) 2018-2022, NVIDIA CORPORATION.
 ##############################################
 # cuDF GPU build and test script for CI      #
 ##############################################
@@ -249,15 +249,15 @@ fi
 
 cd "$WORKSPACE/python/cudf"
 gpuci_logger "Python py.test for cuDF"
-py.test -n 8 --cache-clear --basetemp="$WORKSPACE/cudf-cuda-tmp" --ignore="$WORKSPACE/python/cudf/cudf/benchmarks" --junitxml="$WORKSPACE/junit-cudf.xml" -v --cov-config=.coveragerc --cov=cudf --cov-report=xml:"$WORKSPACE/python/cudf/cudf-coverage.xml" --cov-report term --dist=loadscope
+py.test -n 8 --cache-clear --basetemp="$WORKSPACE/cudf-cuda-tmp" --ignore="$WORKSPACE/python/cudf/cudf/benchmarks" --junitxml="$WORKSPACE/junit-cudf.xml" -v --cov-config=.coveragerc --cov=cudf --cov-report=xml:"$WORKSPACE/python/cudf/cudf-coverage.xml" --cov-report term --dist=loadscope cudf
 
 cd "$WORKSPACE/python/dask_cudf"
 gpuci_logger "Python py.test for dask-cudf"
-py.test -n 8 --cache-clear --basetemp="$WORKSPACE/dask-cudf-cuda-tmp" --junitxml="$WORKSPACE/junit-dask-cudf.xml" -v --cov-config=.coveragerc --cov=dask_cudf --cov-report=xml:"$WORKSPACE/python/dask_cudf/dask-cudf-coverage.xml" --cov-report term
+py.test -n 8 --cache-clear --basetemp="$WORKSPACE/dask-cudf-cuda-tmp" --junitxml="$WORKSPACE/junit-dask-cudf.xml" -v --cov-config=.coveragerc --cov=dask_cudf --cov-report=xml:"$WORKSPACE/python/dask_cudf/dask-cudf-coverage.xml" --cov-report term dask_cudf
 
 cd "$WORKSPACE/python/custreamz"
 gpuci_logger "Python py.test for cuStreamz"
-py.test -n 8 --cache-clear --basetemp="$WORKSPACE/custreamz-cuda-tmp" --junitxml="$WORKSPACE/junit-custreamz.xml" -v --cov-config=.coveragerc --cov=custreamz --cov-report=xml:"$WORKSPACE/python/custreamz/custreamz-coverage.xml" --cov-report term
+py.test -n 8 --cache-clear --basetemp="$WORKSPACE/custreamz-cuda-tmp" --junitxml="$WORKSPACE/junit-custreamz.xml" -v --cov-config=.coveragerc --cov=custreamz --cov-report=xml:"$WORKSPACE/python/custreamz/custreamz-coverage.xml" --cov-report term custreamz
 
 gpuci_logger "Test notebooks"
 "$WORKSPACE/ci/gpu/test-notebooks.sh" 2>&1 | tee nbtest.log
diff --git a/python/cudf/cudf/tests/conftest.py b/python/cudf/cudf/tests/conftest.py
index 041bd055f0a..4d5b5926d6e 100644
--- a/python/cudf/cudf/tests/conftest.py
+++ b/python/cudf/cudf/tests/conftest.py
@@ -1,10 +1,41 @@
+# Copyright (c) 2019-2022, NVIDIA CORPORATION.
+
+import os
 import pathlib
 
 import pytest
 
 import rmm  # noqa: F401
 
+_CURRENT_DIRECTORY = str(pathlib.Path(__file__).resolve().parent)
+
 
 @pytest.fixture(scope="session")
 def datadir():
     return pathlib.Path(__file__).parent / "data"
+
+
+# To set and remove the NO_EXTERNAL_ONLY_APIS environment variable we must use
+# the sessionstart and sessionfinish hooks rather than a simple autouse,
+# session-scope fixture because we need to set these variable before collection
+# occurs because the environment variable will be checked as soon as cudf is
+# imported anywhere.
+def pytest_sessionstart(session):
+    """
+    Called after the Session object has been created and
+    before performing collection and entering the run test loop.
+    """
+    os.environ["NO_EXTERNAL_ONLY_APIS"] = "1"
+    os.environ["_CUDF_TEST_ROOT"] = _CURRENT_DIRECTORY
+
+
+def pytest_sessionfinish(session, exitstatus):
+    """
+    Called after whole test run finished, right before
+    returning the exit status to the system.
+    """
+    try:
+        del os.environ["NO_EXTERNAL_ONLY_APIS"]
+        del os.environ["_CUDF_TEST_ROOT"]
+    except KeyError:
+        pass
diff --git a/python/cudf/cudf/utils/utils.py b/python/cudf/cudf/utils/utils.py
index 4143cbd1d66..b283f89873d 100644
--- a/python/cudf/cudf/utils/utils.py
+++ b/python/cudf/cudf/utils/utils.py
@@ -2,6 +2,8 @@
 
 import decimal
 import functools
+import os
+import traceback
 from collections.abc import Sequence
 from typing import FrozenSet, Set, Union
 
@@ -37,6 +39,60 @@
 }
 
 
+# The test root is set by pytest to support situations where tests are run from
+# a source tree on a built version of cudf.
+NO_EXTERNAL_ONLY_APIS = os.getenv("NO_EXTERNAL_ONLY_APIS")
+
+_cudf_root = os.path.dirname(cudf.__file__)
+# If the environment variable for the test root is not set, we default to
+# using the path relative to the cudf root directory.
+_tests_root = os.getenv("_CUDF_TEST_ROOT") or os.path.join(_cudf_root, "tests")
+
+
+def _external_only_api(func, alternative=""):
+    """Decorator to indicate that a function should not be used internally.
+
+    cudf contains many APIs that exist for pandas compatibility but are
+    intrinsically inefficient. For some of these cudf has internal
+    equivalents that are much faster. Usage of the slow public APIs inside
+    our implementation can lead to unnecessary performance bottlenecks.
+    Applying this decorator to such functions and setting the environment
+    variable NO_EXTERNAL_ONLY_APIS will cause such functions to raise
+    exceptions if they are called from anywhere inside cudf, making it easy
+    to identify and excise such usage.
+
+    The `alternative` should be a complete phrase or sentence since it will
+    be used verbatim in error messages.
+    """
+
+    # If the first arg is a string then an alternative function to use in
+    # place of this API was provided, so we pass that to a subsequent call.
+    # It would be cleaner to implement this pattern by using a class
+    # decorator with a factory method, but there is no way to generically
+    # wrap docstrings on a class (we would need the docstring to be on the
+    # class itself, not instances, because that's what `help` looks at) and
+    # there is also no way to make mypy happy with that approach.
+    if isinstance(func, str):
+        return lambda actual_func: _external_only_api(actual_func, func)
+
+    if not NO_EXTERNAL_ONLY_APIS:
+        return func
+
+    @functools.wraps(func)
+    def wrapper(*args, **kwargs):
+        # Check the immediately preceding frame to see if it's in cudf.
+        frame, lineno = next(traceback.walk_stack(None))
+        fn = frame.f_code.co_filename
+        if _cudf_root in fn and _tests_root not in fn:
+            raise RuntimeError(
+                f"External-only API called in {fn} at line {lineno}. "
+                f"{alternative}"
+            )
+        return func(*args, **kwargs)
+
+    return wrapper
+
+
 def scalar_broadcast_to(scalar, size, dtype=None):
 
     if isinstance(size, (tuple, list)):