diff --git a/lib/validator.js b/lib/validator.js
index 77bbd101..7bec3394 100644
--- a/lib/validator.js
+++ b/lib/validator.js
@@ -43,7 +43,7 @@ class Validator {
       const ajvSchema = clone(schema)
       this.convertSchemaToAjvFormat(ajvSchema)
       this.ajv.addSchema(ajvSchema, schemaKey)
-      this._ajvSchemas[schemaKey] = schema
+      this._ajvSchemas[schemaKey] = ajvSchema
     }
   }
 
@@ -70,6 +70,14 @@ class Validator {
       schema.fjs_type = 'string'
       schema.type.push('object')
     }
+
+    // Ajv percent-decodes the $ref fragment before matching it against the
+    // definitions keys (see ajv's unescapeFragment), while our internal
+    // serializer resolves refs literally. Decode the definitions/$defs keys
+    // so that keys copied verbatim into $ref (e.g. 'Some%3Cloremipsum%3E')
+    // resolve consistently on both sides.
+    this.normalizeDefinitionsKeys(schema)
+
     for (const property in schema) {
       if (typeof schema[property] === 'object') {
         this.convertSchemaToAjvFormat(schema[property])
@@ -77,6 +85,41 @@ class Validator {
     }
   }
 
+  normalizeDefinitionsKeys (schema) {
+    for (const containerKey of ['definitions', '$defs']) {
+      const defs = schema[containerKey]
+      if (defs === null || typeof defs !== 'object' || Array.isArray(defs)) {
+        continue
+      }
+      for (const key of Object.keys(defs)) {
+        let decoded
+        try {
+          decoded = decodeURIComponent(key)
+        } catch {
+          // Invalid or incomplete percent sequences: decode only the valid
+          // percent-encoded segments, leaving the rest untouched.
+          decoded = key.replace(/%[0-9a-fA-F]{2}/g, (segment) => {
+            try {
+              return decodeURIComponent(segment)
+            } catch {
+              return segment
+            }
+          })
+        }
+        if (decoded === key) {
+          continue
+        }
+        // Skip when decoding would collide with an existing key, otherwise
+        // one of the two definitions would silently shadow the other.
+        if (Object.prototype.hasOwnProperty.call(defs, decoded)) {
+          continue
+        }
+        defs[decoded] = defs[key]
+        delete defs[key]
+      }
+    }
+  }
+
   getState () {
     return {
       ajvOptions: this._ajvOptions,
diff --git a/test/ref.test.js b/test/ref.test.js
index 2f736445..1ae830d1 100644
--- a/test/ref.test.js
+++ b/test/ref.test.js
@@ -2075,3 +2075,76 @@ test('ref nested', (t) => {
   t.assert.doesNotThrow(() => JSON.parse(output))
   t.assert.equal(output, '{"str":"test"}')
 })
+
+test('ref internal - definitions key with percent-encoded characters (#740)', (t) => {
+  t.plan(2)
+
+  const schema = {
+    title: 'object with $ref',
+    definitions: {
+      'Some%3Cloremipsum%3E': {
+        additionalProperties: {
+          oneOf: [
+            { type: 'string' },
+            { type: 'number' },
+            { type: 'object' },
+            { type: 'null' }
+          ]
+        },
+        type: 'object'
+      }
+    },
+    type: 'object',
+    properties: {
+      obj: {
+        $ref: '#/definitions/Some%3Cloremipsum%3E'
+      }
+    }
+  }
+
+  const object = {
+    obj: {
+      str: 'test'
+    }
+  }
+
+  const stringify = build(schema)
+  const output = stringify(object)
+
+  t.assert.doesNotThrow(() => JSON.parse(output))
+  t.assert.equal(output, '{"obj":{"str":"test"}}')
+})
+
+test('ref internal - $defs key with percent-encoded characters (#740)', (t) => {
+  t.plan(2)
+
+  const schema = {
+    $defs: {
+      'Some%3Cloremipsum%3E': {
+        type: 'object',
+        properties: {
+          str: { type: 'string' }
+        },
+        required: ['str']
+      }
+    },
+    type: 'object',
+    properties: {
+      obj: {
+        $ref: '#/$defs/Some%3Cloremipsum%3E'
+      }
+    }
+  }
+
+  const object = {
+    obj: {
+      str: 'test'
+    }
+  }
+
+  const stringify = build(schema)
+  const output = stringify(object)
+
+  t.assert.doesNotThrow(() => JSON.parse(output))
+  t.assert.equal(output, '{"obj":{"str":"test"}}')
+})