diff --git a/manifests/native.json b/manifests/native.json
index b21c31a8..f727fd9c 100644
--- a/manifests/native.json
+++ b/manifests/native.json
@@ -2402,6 +2402,11 @@
       "moduleName": "repeat-string",
       "replacements": ["String.prototype.repeat"]
     },
+    "safe-buffer": {
+      "type": "module",
+      "moduleName": "safe-buffer",
+      "replacements": ["safe-buffer"]
+    },
     "setprototypeof": {
       "type": "module",
       "moduleName": "setprototypeof",
diff --git a/scripts/validate-manifests.js b/scripts/validate-manifests.js
index e2147ee9..158c61c6 100644
--- a/scripts/validate-manifests.js
+++ b/scripts/validate-manifests.js
@@ -55,6 +55,8 @@ export async function validateManifests() {
       }
     }
 
+    const usedReplacementIds = new Set();
+
     for (const [key, mapping] of Object.entries(manifest.mappings)) {
       if (mapping.type === 'module' && mapping.moduleName !== key) {
         throw new Error(
@@ -68,6 +70,15 @@ export async function validateManifests() {
             `${manifestPath}: mapping "${key}" references unknown replacement "${replacementId}"`
           );
         }
+        usedReplacementIds.add(replacementId);
+      }
+    }
+
+    for (const id of Object.keys(manifest.replacements)) {
+      if (!usedReplacementIds.has(id)) {
+        throw new Error(
+          `${manifestPath}: replacement "${id}" is defined but not used by any mapping.`
+        );
       }
     }
   }