diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 524b2e0b8..19f7e59e6 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -93,6 +93,12 @@ repos:
         types: [c++]
         language: script
 
+      - id: fix-pragma-once
+        name: fix missing '#pragma once' declarations in header files
+        language: python
+        entry: ./pre-commit-hooks/fix_pragma_once.py
+        files: \.(h|hpp)$
+
   - repo: https://github.com/pre-commit/mirrors-clang-format
     rev: dd18dad857d6133e90bbe478f4f2f22ec0030269 # frozen: v22.1.5
     hooks:
diff --git a/pre-commit-hooks/fix_pragma_once.py b/pre-commit-hooks/fix_pragma_once.py
new file mode 100755
index 000000000..08a505b6d
--- /dev/null
+++ b/pre-commit-hooks/fix_pragma_once.py
@@ -0,0 +1,34 @@
+#!/usr/bin/env python3
+
+"""
+Adds "#pragma once" to the top of header files that don't already have it.
+
+Usage: ./bin/pre-commit/fix_pragma_once.py <file1> <file2> ...
+"""
+
+import sys
+from pathlib import Path
+
+PRAGMA_ONCE = "#pragma once\n\n"
+
+
+def fix_pragma_once(path: Path) -> bool:
+    original = path.read_text(encoding="utf-8")
+    if PRAGMA_ONCE not in original:
+        path.write_text(PRAGMA_ONCE + original, encoding="utf-8")
+        return False
+    return True
+
+
+def main() -> int:
+    files = [Path(f) for f in sys.argv[1:]]
+    success = True
+
+    for path in files:
+        success &= fix_pragma_once(path)
+
+    return 0 if success else 1
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/src/rpc/handlers/LedgerIndex.hpp b/src/rpc/handlers/LedgerIndex.hpp
index 65c709c55..682fadc0e 100644
--- a/src/rpc/handlers/LedgerIndex.hpp
+++ b/src/rpc/handlers/LedgerIndex.hpp
@@ -1,4 +1,5 @@
 #pragma once
+
 #include "data/BackendInterface.hpp"
 #include "rpc/JS.hpp"
 #include "rpc/common/Specs.hpp"
diff --git a/tests/common/data/cassandra/FakesAndMocks.hpp b/tests/common/data/cassandra/FakesAndMocks.hpp
index 4e805751d..2f36e5c2e 100644
--- a/tests/common/data/cassandra/FakesAndMocks.hpp
+++ b/tests/common/data/cassandra/FakesAndMocks.hpp
@@ -1,3 +1,5 @@
+#pragma once
+
 #include "data/cassandra/Error.hpp"
 #include "data/cassandra/impl/AsyncExecutor.hpp"
 
diff --git a/tests/common/migration/TestMigrators.hpp b/tests/common/migration/TestMigrators.hpp
index 82a0fd5e9..290d534e3 100644
--- a/tests/common/migration/TestMigrators.hpp
+++ b/tests/common/migration/TestMigrators.hpp
@@ -1,3 +1,5 @@
+#pragma once
+
 #include "util/MockMigrationBackend.hpp"
 #include "util/config/ObjectView.hpp"
 
diff --git a/tests/common/util/LoggerBuffer.hpp b/tests/common/util/LoggerBuffer.hpp
index 8c62f66a7..138736f56 100644
--- a/tests/common/util/LoggerBuffer.hpp
+++ b/tests/common/util/LoggerBuffer.hpp
@@ -1,4 +1,5 @@
 #pragma once
+
 #include "util/StringBuffer.hpp"
 
 #include <ostream>
diff --git a/tests/common/util/MockRPCEngine.hpp b/tests/common/util/MockRPCEngine.hpp
index c395ecaa1..90fb35808 100644
--- a/tests/common/util/MockRPCEngine.hpp
+++ b/tests/common/util/MockRPCEngine.hpp
@@ -1,4 +1,5 @@
 #pragma once
+
 #include "rpc/common/Types.hpp"
 #include "util/Spawn.hpp"
 #include "web/Context.hpp"
diff --git a/tests/common/util/MockRandomGenerator.hpp b/tests/common/util/MockRandomGenerator.hpp
index f2f124dbb..b9f5eacf4 100644
--- a/tests/common/util/MockRandomGenerator.hpp
+++ b/tests/common/util/MockRandomGenerator.hpp
@@ -1,4 +1,5 @@
 #pragma once
+
 #include "util/Random.hpp"
 
 #include <gtest/gtest.h>