diff --git a/docs/changelog.d/+interrupted-backend.bugfix.rst b/docs/changelog.d/+interrupted-backend.bugfix.rst
new file mode 100644
index 0000000..e3b668e
--- /dev/null
+++ b/docs/changelog.d/+interrupted-backend.bugfix.rst
@@ -0,0 +1 @@
+Stop the backend process and join its output collection thread when a frontend call is interrupted.
diff --git a/src/pyproject_api/_via_fresh_subprocess.py b/src/pyproject_api/_via_fresh_subprocess.py
index e9a2c29..49236d8 100644
--- a/src/pyproject_api/_via_fresh_subprocess.py
+++ b/src/pyproject_api/_via_fresh_subprocess.py
@@ -5,7 +5,7 @@
 from contextlib import contextmanager
 from subprocess import PIPE, Popen  # ruff:ignore[suspicious-subprocess-import]
 from threading import Thread
-from typing import TYPE_CHECKING, Any, cast
+from typing import TYPE_CHECKING, Any, Final, cast
 
 from ._frontend import CmdStatus, Frontend
 
@@ -77,7 +77,12 @@ def _send_msg(self, cmd: str, result_file: Path, msg: str) -> Generator[Subproce
             cwd=self._root,
             env=env,
         )
-        yield SubprocessCmdStatus(process, f"{os.linesep}{msg}{os.linesep}")
+        status: Final = SubprocessCmdStatus(process, f"{os.linesep}{msg}{os.linesep}")
+        try:
+            yield status
+        finally:
+            process.kill()
+            status.join()
 
     def send_cmd(self, cmd: str, **kwargs: Any) -> tuple[Any, str, str]:
         """
diff --git a/tests/test_frontend.py b/tests/test_frontend.py
index 8410d90..5ad3475 100644
--- a/tests/test_frontend.py
+++ b/tests/test_frontend.py
@@ -10,6 +10,7 @@
 import pytest
 from packaging.requirements import Requirement
 
+from pyproject_api import _via_fresh_subprocess
 from pyproject_api._frontend import BackendFailed
 from pyproject_api._via_fresh_subprocess import SubprocessFrontend
 
@@ -161,6 +162,39 @@ def test_large_command_with_failed_backend(local_builder: Callable[[str], Path])
     assert "RuntimeError: backend import failed" in context.value.err
 
 
+@pytest.mark.parametrize(
+    "error_type",
+    [
+        pytest.param(KeyboardInterrupt, id="keyboard-interrupt"),
+        pytest.param(SystemExit, id="system-exit"),
+        pytest.param(RuntimeError, id="runtime-error"),
+    ],
+)
+def test_interrupted_frontend_cleans_up_backend(
+    error_type: type[BaseException], local_builder: Callable[[str], Path], mocker: pytest_mock.MockerFixture
+) -> None:
+    root: Final = local_builder("""
+        import time
+
+        def build_wheel(wheel_directory, config_settings=None, metadata_directory=None):
+            time.sleep(60)
+            return "demo-1.0-py3-none-any.whl"
+        """)
+    status_factory: Final = mocker.spy(_via_fresh_subprocess, "SubprocessCmdStatus")
+    mocker.patch("pyproject_api._frontend.sleep", side_effect=error_type("build interrupted"))
+
+    try:
+        with pytest.raises(error_type):
+            SubprocessFrontend(*SubprocessFrontend.create_args_from_folder(root)[:-1]).build_wheel(root / "dist")
+
+        assert not status_factory.spy_return.is_alive()
+    finally:
+        # Keep a regression from leaving the backend and its non-daemon thread running.
+        status: Final = status_factory.spy_return
+        status.process.kill()
+        status.join(timeout=10)
+
+
 @pytest.mark.parametrize("cmd", ["build_wheel", "build_sdist"])
 def test_missing_required_cmd(cmd: str, local_builder: Callable[[str], Path]) -> None:
     tmp_path = local_builder("")