diff --git a/.semgrep/rules/sol-rules.yaml b/.semgrep/rules/sol-rules.yaml
index 729fb7e199291..3fe1754098d3e 100644
--- a/.semgrep/rules/sol-rules.yaml
+++ b/.semgrep/rules/sol-rules.yaml
@@ -421,3 +421,37 @@ rules:
     paths:
       include:
         - packages/contracts-bedrock/src/L1/opcm/OPContractsManagerV2.sol
+
+  - id: sol-safety-initialize-assert-proxy-admin
+    languages: [solidity]
+    severity: ERROR
+    message: Initialize functions must call _assertOnlyProxyAdminOrProxyAdminOwner() to restrict initialization to authorized callers
+    patterns:
+      - pattern: |
+          function initialize(...) {
+            ...
+          }
+      - pattern-not: |
+          function initialize(...) {
+            ...
+            _assertOnlyProxyAdminOrProxyAdminOwner();
+            ...
+          }
+    paths:
+      include:
+        - packages/contracts-bedrock/src
+      exclude:
+        # TODO(#19001): We should eventually have this on the L2 contracts.
+        - packages/contracts-bedrock/src/L2
+        - packages/contracts-bedrock/src/universal/OptimismMintableERC20Factory.sol
+        # Dispute games use initialize() in a different context - they are clone-deployed
+        # via DisputeGameFactory and use their own AlreadyInitialized check pattern.
+        - packages/contracts-bedrock/src/dispute/FaultDisputeGame.sol
+        - packages/contracts-bedrock/src/dispute/PermissionedDisputeGame.sol
+        - packages/contracts-bedrock/src/dispute/SuperFaultDisputeGame.sol
+        - packages/contracts-bedrock/src/dispute/SuperPermissionedDisputeGame.sol
+        - packages/contracts-bedrock/src/dispute/zk/OptimisticZkGame.sol
+        # ProtocolVersions is being deprecated.
+        - packages/contracts-bedrock/src/L1/ProtocolVersions.sol
+        # DataAvailabilityChallenge is a beta/non-standard contract.
+        - packages/contracts-bedrock/src/L1/DataAvailabilityChallenge.sol
diff --git a/.semgrep/tests/sol-rules.sol-safety-initialize-assert-proxy-admin.t.sol b/.semgrep/tests/sol-rules.sol-safety-initialize-assert-proxy-admin.t.sol
new file mode 100644
index 0000000000000..aee6968dabe34
--- /dev/null
+++ b/.semgrep/tests/sol-rules.sol-safety-initialize-assert-proxy-admin.t.sol
@@ -0,0 +1,45 @@
+// Semgrep tests for Solidity rules are defined in this file.
+// Semgrep tests do not need to be valid Solidity code but should be syntactically correct so that
+// Semgrep can parse them. You don't need to be able to *run* the code here but it should look like
+// the code that you expect to catch with the rule.
+//
+// Semgrep testing 101
+// Use comments like "ruleid: <rule-id>" to assert that the rule catches the code.
+// Use comments like "ok: <rule-id>" to assert that the rule does not catch the code.
+
+contract SemgrepTest__sol_safety_initialize_assert_proxy_admin {
+    // ok: sol-safety-initialize-assert-proxy-admin
+    function initialize(address _param) external reinitializer(1) {
+        _assertOnlyProxyAdminOrProxyAdminOwner();
+        _setParam(_param);
+    }
+
+    // ok: sol-safety-initialize-assert-proxy-admin
+    function initialize() external initializer {
+        _assertOnlyProxyAdminOrProxyAdminOwner();
+    }
+
+    // ok: sol-safety-initialize-assert-proxy-admin
+    function initialize(address _a, uint256 _b) public reinitializer(2) {
+        // Comment before assertion
+        _assertOnlyProxyAdminOrProxyAdminOwner();
+        // More initialization
+        param = _a;
+    }
+
+    // ruleid: sol-safety-initialize-assert-proxy-admin
+    function initialize(address _param) external reinitializer(1) {
+        _setParam(_param);
+    }
+
+    // ruleid: sol-safety-initialize-assert-proxy-admin
+    function initialize() external initializer {
+        __Ownable_init();
+    }
+
+    // ruleid: sol-safety-initialize-assert-proxy-admin
+    function initialize(address _a, uint256 _b) public initializer {
+        param = _a;
+        value = _b;
+    }
+}
diff --git a/.semgrep/tests/sol-rules.sol-safety-proper-initializer.t.sol b/.semgrep/tests/sol-rules.sol-safety-proper-initializer.t.sol
index 23ad245b1e912..fd5fa665369d7 100644
--- a/.semgrep/tests/sol-rules.sol-safety-proper-initializer.t.sol
+++ b/.semgrep/tests/sol-rules.sol-safety-proper-initializer.t.sol
@@ -15,11 +15,13 @@
 contract SemgrepTest__sol_safety_proper_initializer {
     // ok: sol-safety-proper-initializer
     function initialize() external {
+        _assertOnlyProxyAdminOrProxyAdminOwner();
         // ...
     }
 
     // ok: sol-safety-proper-initializer
     function initialize() public {
+        _assertOnlyProxyAdminOrProxyAdminOwner();
         // ...
     }
 }
@@ -29,36 +31,43 @@ contract SemgrepTest__sol_safety_proper_initializer {
 contract SemgrepTest__sol_safety_proper_initializer {
     // ok: sol-safety-proper-initializer
     function initialize() external initializer {
+        _assertOnlyProxyAdminOrProxyAdminOwner();
         // ...
     }
 
     // ok: sol-safety-proper-initializer
     function initialize() public initializer {
+        _assertOnlyProxyAdminOrProxyAdminOwner();
         // ...
     }
 
     // ok: sol-safety-proper-initializer
     function initialize() external reinitializer(1) {
+        _assertOnlyProxyAdminOrProxyAdminOwner();
         // ...
     }
 
     // ok: sol-safety-proper-initializer
     function initialize() external reinitializer(1) {
+        _assertOnlyProxyAdminOrProxyAdminOwner();
         // ...
     }
 
     // ok: sol-safety-proper-initializer
     function initialize() public reinitializer(2) {
+        _assertOnlyProxyAdminOrProxyAdminOwner();
         // ...
     }
 
     // ruleid: sol-safety-proper-initializer
     function initialize() internal {
+        _assertOnlyProxyAdminOrProxyAdminOwner();
         // ...
     }
 
     // ruleid: sol-safety-proper-initializer
     function initialize() public {
+        _assertOnlyProxyAdminOrProxyAdminOwner();
         // ...
     }
 }