diff --git a/pyproject.toml b/pyproject.toml
index 3d683aebe..e9c518e5d 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -59,6 +59,8 @@ markers = [
     "slow",
     # experimental mark for all tests using pexpect
     "uses_pexpect",
+    # runs tests for ./testing/downstream_testing/downstream_runner.py
+    "downstream",
 ]
 
 
diff --git a/setup.cfg b/setup.cfg
index fe6ea4095..a63ec5fcd 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -65,6 +65,7 @@ console_scripts =
 
 [options.extras_require]
 testing =
+    PyYAML
     argcomplete
     hypothesis>=3.56
     mock
diff --git a/testing/downstream_testing/test_downstream_runner.py b/testing/downstream_testing/test_downstream_runner.py
new file mode 100644
index 000000000..1b22ba226
--- /dev/null
+++ b/testing/downstream_testing/test_downstream_runner.py
@@ -0,0 +1,281 @@
+from __future__ import annotations
+
+import sys
+from typing import Any
+
+import pytest
+
+sys.path.append("testing")
+from downstream_testing import downstream_runner  # noqa: E402
+
+xfail = pytest.mark.xfail
+
+DUMMY_YAML_COMBINATION = """---
+name: dummy-include
+jobs:
+  test:
+    runs-on: ubuntu-20.04
+
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+        - name: i-do-not-start-with-py
+          python: 3.6
+          allow_failure: false
+
+        - name: py310-dj40-postgres-xdist-coverage
+          python: '3.10'
+          allow_failure: false
+
+        - name: py36-exclude-me
+          python: 3.6
+          allow_failure: false
+
+        - name: py37-exclude-me
+          python: 3.7
+          allow_failure: false
+"""
+
+DUMMY_YAML_NO_COMBINATION = """---
+name: dummy-no-include
+jobs:
+  test:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, windows-latest]
+        python-version: [3.6, 3.7, 3.8, 3.9, "3.10", pypy3]
+        exclude:
+          - os: windows-latest
+            python-version: pypy3
+"""
+
+
+@pytest.fixture()
+def mock_schema_combination(monkeypatch):
+    def mock_load_schema_combination(repo: str) -> dict[str, Any]:
+        return {
+            "matrix": ["matrix", "include"],
+            "tox_cmd_build": {
+                "base": "name",
+                "prefix": "py",
+                "sub": {"pattern": "-coverage$", "replace": ""},
+            },
+            "python_version": "python",
+        }
+
+    monkeypatch.setattr(
+        downstream_runner, "load_matrix_schema", mock_load_schema_combination
+    )
+
+
+@pytest.fixture()
+def mock_schema_no_combination(monkeypatch):
+    def mock_load_schema_no_combination(repo: str) -> dict[str, Any]:
+        return {
+            "matrix": ["matrix", "python-version"],
+            "tox_cmd_build": {
+                "base": "",
+                "prefix": "",
+                "sub": {"pattern": "(\\d|py\\d)\\.*(\\d+)", "replace": "py\\1\\2"},
+            },
+            "python_version": "python",
+        }
+
+    monkeypatch.setattr(
+        downstream_runner, "load_matrix_schema", mock_load_schema_no_combination
+    )
+
+
+@pytest.mark.downstream
+class TestDownstreamRunnerConstructor:
+    @pytest.mark.parametrize(
+        "args",
+        [
+            pytest.param(tuple(), marks=xfail),
+            pytest.param(("repo",), marks=xfail),
+            pytest.param(("repo", "yaml_source"), marks=xfail),
+            ("pytest-downstream", "yaml_source", ["jobs"]),
+        ],
+        ids=["no args", "repo only", "missing jobs", "all args"],
+    )
+    def test_args(self, args, mock_schema_combination) -> None:
+        downstream_runner.DownstreamRunner(*args)
+
+    @pytest.mark.parametrize(
+        "kwargs",
+        [
+            ("matrix_exclude", "exclude"),
+            ("dry_run", True),
+        ],
+        ids=["matrix_exclude", "dry_run"],
+    )
+    def test_kwargs(self, kwargs, mock_schema_combination) -> None:
+        args = ("pytest-downstream", "yaml_source", ["test"])
+        new_kwargs = {kwargs[0]: kwargs[1]}
+        runner = downstream_runner.DownstreamRunner(*args, **new_kwargs)
+
+        assert kwargs[1] == getattr(runner, kwargs[0])
+
+
+@pytest.mark.downstream
+class TestDownstreamRunnerProperties:
+    def test_yaml_tree_file_doesnt_exist(self, mock_schema_combination) -> None:
+        runner = downstream_runner.DownstreamRunner(
+            "pytest-downstream", "yaml_source", ["test"], dry_run=True
+        )
+
+        with pytest.raises(FileNotFoundError):
+            runner.yaml_tree
+
+    def test_yaml_tree_bad_yaml(self, mock_schema_combination, tmp_path) -> None:
+        yaml_source = tmp_path / "test.yml"
+        yaml_source.write_text("---\n:")
+
+        runner = downstream_runner.DownstreamRunner(
+            "pytest-downstream", yaml_source, ["test"], dry_run=True
+        )
+
+        with pytest.raises(RuntimeError) as excinfo:
+            runner.yaml_tree
+        assert str(yaml_source) in excinfo.exconly(tryshort=True)
+
+    def test_yaml_tree_empty_yaml(self, mock_schema_combination, tmp_path) -> None:
+        yaml_source = tmp_path / "test.yml"
+        yaml_source.write_text("---")
+
+        runner = downstream_runner.DownstreamRunner(
+            "pytest-downstream", yaml_source, ["test"], dry_run=True
+        )
+
+        with pytest.raises(RuntimeError) as excinfo:
+            runner.yaml_tree
+        assert str(yaml_source) in excinfo.exconly(tryshort=True)
+
+    def test_yaml_tree_passing_yaml(self, mock_schema_combination, tmp_path) -> None:
+        yaml_source = tmp_path / "test.yml"
+        yaml_source.write_text(DUMMY_YAML_COMBINATION)
+
+        runner = downstream_runner.DownstreamRunner(
+            "pytest-downstream", yaml_source, ["test"], dry_run=True
+        )
+
+        assert runner.yaml_tree["name"] == "dummy-include"
+        assert "test" in runner.yaml_tree["jobs"]
+
+    def test_matrix_combination(self, mock_schema_combination, tmp_path) -> None:
+        yaml_source = tmp_path / "test.yml"
+        yaml_source.write_text(DUMMY_YAML_COMBINATION)
+
+        runner = downstream_runner.DownstreamRunner(
+            "pytest-downstream", yaml_source, ["test"], dry_run=True
+        )
+
+        assert all(
+            [
+                matrice.get("tox_cmd", "").startswith("py")
+                for matrice in runner.matrix["test"]
+            ]
+        )
+
+    def test_matrix_no_combination(self, mock_schema_no_combination, tmp_path) -> None:
+        yaml_source = tmp_path / "test.yml"
+        yaml_source.write_text(DUMMY_YAML_NO_COMBINATION)
+
+        runner = downstream_runner.DownstreamRunner(
+            "pytest-downstream", yaml_source, ["test"], dry_run=True
+        )
+
+        assert all(
+            [
+                matrice.get("tox_cmd", "").startswith("py")
+                for matrice in runner.matrix["test"]
+            ]
+        )
+
+    def test_matrix_combination_matrix_exclude(
+        self, mock_schema_combination, tmp_path
+    ) -> None:
+        yaml_source = tmp_path / "test.yml"
+        yaml_source.write_text(DUMMY_YAML_COMBINATION)
+        matrix_to_exclude = ["py36-exclude-me", "py37-exclude-me"]
+
+        runner = downstream_runner.DownstreamRunner(
+            "pytest-downstream",
+            yaml_source,
+            ["test"],
+            matrix_exclude=matrix_to_exclude,
+            dry_run=True,
+        )
+
+        matrix_names = {matrice["name"] for matrice in runner.matrix["test"]}
+        assert matrix_names.isdisjoint(set(matrix_to_exclude))
+
+    def test_matrix_no_combination_matrix_exclude(
+        self, mock_schema_no_combination, tmp_path
+    ) -> None:
+        yaml_source = tmp_path / "test.yml"
+        yaml_source.write_text(DUMMY_YAML_NO_COMBINATION)
+        matrix_to_exclude = ["3.6", "3.7"]
+
+        runner = downstream_runner.DownstreamRunner(
+            "pytest-downstream",
+            yaml_source,
+            ["test"],
+            matrix_exclude=matrix_to_exclude,
+            dry_run=True,
+        )
+
+        matrix_names = {matrice["name"] for matrice in runner.matrix["test"]}
+        assert matrix_names.isdisjoint({"py36", "py37"})
+
+
+@pytest.mark.downstream
+class TestDownstreamRunnerBuild:
+    def test_build_run_combination_matrix(
+        self, mock_schema_combination, tmp_path
+    ) -> None:
+        yaml_source = tmp_path / "test.yml"
+        yaml_source.write_text(DUMMY_YAML_COMBINATION)
+        matrix_to_exclude = ["py36-exclude-me", "py37-exclude-me"]
+
+        runner = downstream_runner.DownstreamRunner(
+            "pytest-downstream",
+            yaml_source,
+            ["test"],
+            matrix_exclude=matrix_to_exclude,
+            dry_run=True,
+        )
+
+        run = runner.build_run()
+        assert run == {
+            "py310-dj40-postgres-xdist-coverage": [
+                "pip install tox",
+                "tox -e py310-dj40-postgres-xdist",
+            ]
+        }
+
+    def test_build_run_no_combination_matrix(
+        self, mock_schema_no_combination, tmp_path
+    ) -> None:
+        yaml_source = tmp_path / "test.yml"
+        yaml_source.write_text(DUMMY_YAML_NO_COMBINATION)
+        matrix_to_exclude = ["3.6", "3.7"]
+
+        runner = downstream_runner.DownstreamRunner(
+            "pytest-downstream",
+            yaml_source,
+            ["test"],
+            matrix_exclude=matrix_to_exclude,
+            dry_run=True,
+        )
+
+        run = runner.build_run()
+        assert run == {
+            "py310": ["pip install tox", "tox -e py310"],
+            "py38": ["pip install tox", "tox -e py38"],
+            "py39": ["pip install tox", "tox -e py39"],
+            "pypy3": ["pip install tox", "tox -e pypy3"],
+        }
diff --git a/tox.ini b/tox.ini
index 93c390ffc..9ed36183a 100644
--- a/tox.ini
+++ b/tox.ini
@@ -16,6 +16,7 @@ envlist =
     py37-freeze
     docs
     docs-checklinks
+    downstream
 
 
 
@@ -27,7 +28,7 @@ commands =
     coverage: coverage report -m
 passenv = USER USERNAME COVERAGE_* PYTEST_ADDOPTS TERM SETUPTOOLS_SCM_PRETEND_VERSION_FOR_PYTEST
 setenv =
-    _PYTEST_TOX_DEFAULT_POSARGS={env:_PYTEST_TOX_POSARGS_DOCTESTING:} {env:_PYTEST_TOX_POSARGS_LSOF:} {env:_PYTEST_TOX_POSARGS_XDIST:}
+    _PYTEST_TOX_DEFAULT_POSARGS={env:_PYTEST_TOX_POSARGS_DOCTESTING:} {env:_PYTEST_TOX_POSARGS_LSOF:} {env:_PYTEST_TOX_POSARGS_XDIST:} {env:_PYTEST_TOX_POSARGS_DOWNSTREAM:}
 
     # Configuration to run with coverage similar to CI, e.g.
     # "tox -e py37-coverage".
@@ -43,6 +44,8 @@ setenv =
     lsof: _PYTEST_TOX_POSARGS_LSOF=--lsof
 
     xdist: _PYTEST_TOX_POSARGS_XDIST=-n auto
+
+    downstream: _PYTEST_TOX_POSARGS_DOWNSTREAM=-m downstream
 extras = testing
 deps =
     doctesting: PyYAML