diff --git a/src/plugins/flow.js b/src/plugins/flow.js
index ef9a73a073..f74de36944 100644
--- a/src/plugins/flow.js
+++ b/src/plugins/flow.js
@@ -1049,6 +1049,18 @@ export default function (instance) {
     };
   });
 
+  instance.extend("parseMaybeDefault", function (inner) {
+    return function (...args) {
+      const node = inner.apply(this, args);
+
+      if (node.type === "AssignmentPattern" && node.typeAnnotation && node.right.start < node.typeAnnotation.start) {
+        this.raise(node.typeAnnotation.start, "Type annotations must come before default assignments, e.g. instead of `age = 25: number` use `age: number = 25`");
+      }
+
+      return node;
+    };
+  });
+
 
   // parse typeof and type imports
   instance.extend("parseImportSpecifiers", function (inner) {
diff --git a/test/fixtures/flow/type-annotations/105/actual.js b/test/fixtures/flow/type-annotations/105/actual.js
index 457dd3366d..5e860ef08e 100644
--- a/test/fixtures/flow/type-annotations/105/actual.js
+++ b/test/fixtures/flow/type-annotations/105/actual.js
@@ -1 +1 @@
-function g(a:number=1, e=1:number) {}
+function g(a:number=1, e:number=1) {}
diff --git a/test/fixtures/flow/type-annotations/105/expected.json b/test/fixtures/flow/type-annotations/105/expected.json
index 48c65c3448..a8d2cfdcfd 100644
--- a/test/fixtures/flow/type-annotations/105/expected.json
+++ b/test/fixtures/flow/type-annotations/105/expected.json
@@ -54,7 +54,8 @@
             "end": {
               "line": 1,
               "column": 10
-            }
+            },
+            "identifierName": "g"
           },
           "name": "g"
         },
@@ -88,7 +89,8 @@
                 "end": {
                   "line": 1,
                   "column": 19
-                }
+                },
+                "identifierName": "a"
               },
               "name": "a",
               "typeAnnotation": {
@@ -160,7 +162,7 @@
             "left": {
               "type": "Identifier",
               "start": 23,
-              "end": 24,
+              "end": 31,
               "loc": {
                 "start": {
                   "line": 1,
@@ -168,23 +170,54 @@
                 },
                 "end": {
                   "line": 1,
-                  "column": 24
-                }
+                  "column": 31
+                },
+                "identifierName": "e"
               },
-              "name": "e"
+              "name": "e",
+              "typeAnnotation": {
+                "type": "TypeAnnotation",
+                "start": 24,
+                "end": 31,
+                "loc": {
+                  "start": {
+                    "line": 1,
+                    "column": 24
+                  },
+                  "end": {
+                    "line": 1,
+                    "column": 31
+                  }
+                },
+                "typeAnnotation": {
+                  "type": "NumberTypeAnnotation",
+                  "start": 25,
+                  "end": 31,
+                  "loc": {
+                    "start": {
+                      "line": 1,
+                      "column": 25
+                    },
+                    "end": {
+                      "line": 1,
+                      "column": 31
+                    }
+                  }
+                }
+              }
             },
             "right": {
               "type": "NumericLiteral",
-              "start": 25,
-              "end": 26,
+              "start": 32,
+              "end": 33,
               "loc": {
                 "start": {
                   "line": 1,
-                  "column": 25
+                  "column": 32
                 },
                 "end": {
                   "line": 1,
-                  "column": 26
+                  "column": 33
                 }
               },
               "extra": {
@@ -192,36 +225,6 @@
                 "raw": "1"
               },
               "value": 1
-            },
-            "typeAnnotation": {
-              "type": "TypeAnnotation",
-              "start": 26,
-              "end": 33,
-              "loc": {
-                "start": {
-                  "line": 1,
-                  "column": 26
-                },
-                "end": {
-                  "line": 1,
-                  "column": 33
-                }
-              },
-              "typeAnnotation": {
-                "type": "NumberTypeAnnotation",
-                "start": 27,
-                "end": 33,
-                "loc": {
-                  "start": {
-                    "line": 1,
-                    "column": 27
-                  },
-                  "end": {
-                    "line": 1,
-                    "column": 33
-                  }
-                }
-              }
             }
           }
         ],
@@ -246,4 +249,4 @@
     ],
     "directives": []
   }
-}
+}
\ No newline at end of file
diff --git a/test/fixtures/flow/type-annotations/with-default-invalid/actual.js b/test/fixtures/flow/type-annotations/with-default-invalid/actual.js
new file mode 100644
index 0000000000..93f01d1350
--- /dev/null
+++ b/test/fixtures/flow/type-annotations/with-default-invalid/actual.js
@@ -0,0 +1 @@
+function x(foo = "1": string) {}
\ No newline at end of file
diff --git a/test/fixtures/flow/type-annotations/with-default-invalid/options.json b/test/fixtures/flow/type-annotations/with-default-invalid/options.json
new file mode 100644
index 0000000000..95ca28398b
--- /dev/null
+++ b/test/fixtures/flow/type-annotations/with-default-invalid/options.json
@@ -0,0 +1,3 @@
+{
+	"throws": "Type annotations must come before default assignments, e.g. instead of `age = 25: number` use `age: number = 25` (1:20)"
+}
\ No newline at end of file
diff --git a/test/fixtures/flow/type-annotations/with-default-valid/actual.js b/test/fixtures/flow/type-annotations/with-default-valid/actual.js
new file mode 100644
index 0000000000..d35b6a534c
--- /dev/null
+++ b/test/fixtures/flow/type-annotations/with-default-valid/actual.js
@@ -0,0 +1 @@
+function x(foo: string = "1") {}
diff --git a/test/fixtures/flow/type-annotations/with-default-valid/expected.json b/test/fixtures/flow/type-annotations/with-default-valid/expected.json
new file mode 100644
index 0000000000..49e6fd1fae
--- /dev/null
+++ b/test/fixtures/flow/type-annotations/with-default-valid/expected.json
@@ -0,0 +1,170 @@
+{
+  "type": "File",
+  "start": 0,
+  "end": 32,
+  "loc": {
+    "start": {
+      "line": 1,
+      "column": 0
+    },
+    "end": {
+      "line": 1,
+      "column": 32
+    }
+  },
+  "program": {
+    "type": "Program",
+    "start": 0,
+    "end": 32,
+    "loc": {
+      "start": {
+        "line": 1,
+        "column": 0
+      },
+      "end": {
+        "line": 1,
+        "column": 32
+      }
+    },
+    "sourceType": "module",
+    "body": [
+      {
+        "type": "FunctionDeclaration",
+        "start": 0,
+        "end": 32,
+        "loc": {
+          "start": {
+            "line": 1,
+            "column": 0
+          },
+          "end": {
+            "line": 1,
+            "column": 32
+          }
+        },
+        "id": {
+          "type": "Identifier",
+          "start": 9,
+          "end": 10,
+          "loc": {
+            "start": {
+              "line": 1,
+              "column": 9
+            },
+            "end": {
+              "line": 1,
+              "column": 10
+            },
+            "identifierName": "x"
+          },
+          "name": "x"
+        },
+        "generator": false,
+        "expression": false,
+        "async": false,
+        "params": [
+          {
+            "type": "AssignmentPattern",
+            "start": 11,
+            "end": 28,
+            "loc": {
+              "start": {
+                "line": 1,
+                "column": 11
+              },
+              "end": {
+                "line": 1,
+                "column": 28
+              }
+            },
+            "left": {
+              "type": "Identifier",
+              "start": 11,
+              "end": 22,
+              "loc": {
+                "start": {
+                  "line": 1,
+                  "column": 11
+                },
+                "end": {
+                  "line": 1,
+                  "column": 22
+                },
+                "identifierName": "foo"
+              },
+              "name": "foo",
+              "typeAnnotation": {
+                "type": "TypeAnnotation",
+                "start": 14,
+                "end": 22,
+                "loc": {
+                  "start": {
+                    "line": 1,
+                    "column": 14
+                  },
+                  "end": {
+                    "line": 1,
+                    "column": 22
+                  }
+                },
+                "typeAnnotation": {
+                  "type": "StringTypeAnnotation",
+                  "start": 16,
+                  "end": 22,
+                  "loc": {
+                    "start": {
+                      "line": 1,
+                      "column": 16
+                    },
+                    "end": {
+                      "line": 1,
+                      "column": 22
+                    }
+                  }
+                }
+              }
+            },
+            "right": {
+              "type": "StringLiteral",
+              "start": 25,
+              "end": 28,
+              "loc": {
+                "start": {
+                  "line": 1,
+                  "column": 25
+                },
+                "end": {
+                  "line": 1,
+                  "column": 28
+                }
+              },
+              "extra": {
+                "rawValue": "1",
+                "raw": "\"1\""
+              },
+              "value": "1"
+            }
+          }
+        ],
+        "body": {
+          "type": "BlockStatement",
+          "start": 30,
+          "end": 32,
+          "loc": {
+            "start": {
+              "line": 1,
+              "column": 30
+            },
+            "end": {
+              "line": 1,
+              "column": 32
+            }
+          },
+          "body": [],
+          "directives": []
+        }
+      }
+    ],
+    "directives": []
+  }
+}
\ No newline at end of file