diff --git a/template/.pre-commit-config.yaml.j2 b/template/.pre-commit-config.yaml.j2
index 9b181f8a..33206497 100644
--- a/template/.pre-commit-config.yaml.j2
+++ b/template/.pre-commit-config.yaml.j2
@@ -66,6 +66,12 @@ repos:
 
   - repo: local
     hooks:
+      - id: check-namespaced-defines
+        name: check-namespaced-defines
+        language: system
+        entry: python3 scripts/check_namespaced_defines.py
+        files: ^deploy/helm/[^/]+/templates/
+
       - id: regenerate-charts
         name: regenerate-charts
         language: system
diff --git a/template/scripts/check_namespaced_defines.py b/template/scripts/check_namespaced_defines.py
new file mode 100644
index 00000000..679cae73
--- /dev/null
+++ b/template/scripts/check_namespaced_defines.py
@@ -0,0 +1,93 @@
+"""
+Fails when a Helm defined template, or a call to one, is not prefixed with the
+name of the chart it lives in.
+
+To run tests for this script:
+    python3 -m unittest check_namespaced_defines.py
+"""
+
+import re
+import unittest
+from pathlib import Path
+
+NAME_PATTERN = r'\b(?:define|include|template)\s+"([^"]+)"'
+CHART_PATTERN = r"(?:^|/)deploy/helm/([^/]+)/templates/"
+
+
+def chart_of(path):
+    """The chart whose templates directory holds path, or None if it is outside one."""
+    found = re.search(CHART_PATTERN, Path(path).as_posix())
+    return found.group(1) if found else None
+
+
+def unprefixed(text, chart):
+    """The defined template names in text, and the calls to them, lacking the chart prefix."""
+    names = re.findall(NAME_PATTERN, text)
+    return sorted({name for name in names if not name.startswith(f"{chart}.")})
+
+
+class TestCoreMethods(unittest.TestCase):
+    def test_chart_of(self):
+        self.assertEqual(
+            chart_of("deploy/helm/trino-operator/templates/x.yaml"), "trino-operator"
+        )
+        # An absolute path has to give the same answer, or a manual run invents a chart name.
+        self.assertEqual(
+            chart_of("/tmp/wt/deploy/helm/secret-operator/templates/a/b.yaml"),
+            "secret-operator",
+        )
+        self.assertIsNone(chart_of("deploy/helm/trino-operator/values.yaml"))
+        self.assertIsNone(chart_of("README.md"))
+
+    def test_prefixed_is_accepted(self):
+        text = '{{- define "trino-operator.labels" -}}\n{{ include "trino-operator.chart" . }}\n'
+        self.assertEqual(unprefixed(text, "trino-operator"), [])
+
+    def test_unprefixed_define_is_reported(self):
+        self.assertEqual(
+            unprefixed('{{- define "helper.thing" -}}', "trino-operator"),
+            ["helper.thing"],
+        )
+
+    def test_unprefixed_call_is_reported(self):
+        # A renamed definition whose call sites did not move with it stops the chart rendering.
+        text = (
+            '{{ include "operator.fullname" . }}\n{{ template "operator.labels" . }}\n'
+        )
+        self.assertEqual(
+            unprefixed(text, "trino-operator"), ["operator.fullname", "operator.labels"]
+        )
+
+    def test_keyword_must_stand_alone(self):
+        # Without a word boundary any identifier ending in the keyword matches.
+        self.assertEqual(
+            unprefixed('{{ .Values.xinclude "bar.baz" }}', "foo-operator"), []
+        )
+
+
+if __name__ == "__main__":
+    import sys
+
+    if not sys.argv[1:]:
+        print(f"usage: {sys.argv[0]} deploy/helm/<chart>/templates/*", file=sys.stderr)
+        sys.exit(2)
+
+    failed = False
+    for path in sys.argv[1:]:
+        chart = chart_of(path)
+        if chart is None:
+            print(
+                f"{sys.argv[0]}: {path} is not inside deploy/helm/<chart>/templates/",
+                file=sys.stderr,
+            )
+            sys.exit(2)
+        names = unprefixed(Path(path).read_text(), chart)
+        if names:
+            failed = True
+            print(
+                f"{path}: defined templates and the calls to them must be prefixed with '{chart}.'"
+            )
+            for name in names:
+                print(f"  {name}")
+
+    sys.exit(1 if failed else 0)