diff --git a/packages/babel-parser/src/plugins/typescript/index.js b/packages/babel-parser/src/plugins/typescript/index.js
index 785639ead9..d2f3f334c1 100644
--- a/packages/babel-parser/src/plugins/typescript/index.js
+++ b/packages/babel-parser/src/plugins/typescript/index.js
@@ -69,6 +69,8 @@ const TSErrors = Object.freeze({
     "Type parameters cannot appear on a constructor declaration.",
   DeclareClassFieldHasInitializer:
     "'declare' class fields cannot have an initializer",
+  DeclareFunctionHasImplementation:
+    "An implementation cannot be declared in ambient contexts.",
   DuplicateModifier: "Duplicate modifier: '%0'",
   EmptyHeritageClauseType: "'%0' list cannot be empty.",
   IndexSignatureHasAbstract:
@@ -1469,41 +1471,49 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         kind = "let";
       }
 
-      switch (starttype) {
-        case tt._function:
-          return this.parseFunctionStatement(
-            nany,
-            /* async */ false,
-            /* declarationPosition */ true,
-          );
-        case tt._class:
-          // While this is also set by tsParseExpressionStatement, we need to set it
-          // before parsing the class declaration to now how to register it in the scope.
-          nany.declare = true;
-          return this.parseClass(
-            nany,
-            /* isStatement */ true,
-            /* optionalId */ false,
-          );
-        case tt._const:
-          if (this.match(tt._const) && this.isLookaheadContextual("enum")) {
-            // `const enum = 0;` not allowed because "enum" is a strict mode reserved word.
-            this.expect(tt._const);
-            this.expectContextual("enum");
-            return this.tsParseEnumDeclaration(nany, /* isConst */ true);
-          }
-        // falls through
-        case tt._var:
-          kind = kind || this.state.value;
-          return this.parseVarStatement(nany, kind);
-        case tt.name: {
-          const value = this.state.value;
-          if (value === "global") {
-            return this.tsParseAmbientExternalModuleDeclaration(nany);
-          } else {
-            return this.tsParseDeclaration(nany, value, /* next */ true);
+      const oldIsDeclareContext = this.state.isDeclareContext;
+      this.state.isDeclareContext = true;
+
+      try {
+        switch (starttype) {
+          case tt._function:
+            nany.declare = true;
+            return this.parseFunctionStatement(
+              nany,
+              /* async */ false,
+              /* declarationPosition */ true,
+            );
+          case tt._class:
+            // While this is also set by tsParseExpressionStatement, we need to set it
+            // before parsing the class declaration to now how to register it in the scope.
+            nany.declare = true;
+            return this.parseClass(
+              nany,
+              /* isStatement */ true,
+              /* optionalId */ false,
+            );
+          case tt._const:
+            if (this.match(tt._const) && this.isLookaheadContextual("enum")) {
+              // `const enum = 0;` not allowed because "enum" is a strict mode reserved word.
+              this.expect(tt._const);
+              this.expectContextual("enum");
+              return this.tsParseEnumDeclaration(nany, /* isConst */ true);
+            }
+          // falls through
+          case tt._var:
+            kind = kind || this.state.value;
+            return this.parseVarStatement(nany, kind);
+          case tt.name: {
+            const value = this.state.value;
+            if (value === "global") {
+              return this.tsParseAmbientExternalModuleDeclaration(nany);
+            } else {
+              return this.tsParseDeclaration(nany, value, /* next */ true);
+            }
           }
         }
+      } finally {
+        this.state.isDeclareContext = oldIsDeclareContext;
       }
     }
 
@@ -1764,6 +1774,16 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         this.finishNode(node, bodilessType);
         return;
       }
+      if (bodilessType === "TSDeclareFunction" && this.state.isDeclareContext) {
+        this.raise(node.start, TSErrors.DeclareFunctionHasImplementation);
+        if (
+          // $FlowIgnore
+          node.declare
+        ) {
+          super.parseFunctionBodyAndFinish(node, bodilessType, isMethod);
+          return;
+        }
+      }
 
       super.parseFunctionBodyAndFinish(node, type, isMethod);
     }
diff --git a/packages/babel-parser/src/tokenizer/state.js b/packages/babel-parser/src/tokenizer/state.js
index c9e46a3c96..730be2a577 100644
--- a/packages/babel-parser/src/tokenizer/state.js
+++ b/packages/babel-parser/src/tokenizer/state.js
@@ -70,6 +70,7 @@ export default class State {
   inPropertyName: boolean = false;
   hasFlowComment: boolean = false;
   isIterator: boolean = false;
+  isDeclareContext: boolean = false;
 
   // For the smartPipelines plugin:
   topicContext: TopicContextState = {
diff --git a/packages/babel-parser/test/fixtures/typescript/declare/function/input.ts b/packages/babel-parser/test/fixtures/typescript/declare/function/input.ts
new file mode 100644
index 0000000000..83677c6aa0
--- /dev/null
+++ b/packages/babel-parser/test/fixtures/typescript/declare/function/input.ts
@@ -0,0 +1 @@
+declare function foo() {}
diff --git a/packages/babel-parser/test/fixtures/typescript/declare/function/output.json b/packages/babel-parser/test/fixtures/typescript/declare/function/output.json
new file mode 100644
index 0000000000..490c734868
--- /dev/null
+++ b/packages/babel-parser/test/fixtures/typescript/declare/function/output.json
@@ -0,0 +1,35 @@
+{
+  "type": "File",
+  "start":0,"end":25,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":25}},
+  "errors": [
+    "SyntaxError: An implementation cannot be declared in ambient contexts. (1:0)"
+  ],
+  "program": {
+    "type": "Program",
+    "start":0,"end":25,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":25}},
+    "sourceType": "module",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "TSDeclareFunction",
+        "start":0,"end":25,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":25}},
+        "declare": true,
+        "id": {
+          "type": "Identifier",
+          "start":17,"end":20,"loc":{"start":{"line":1,"column":17},"end":{"line":1,"column":20},"identifierName":"foo"},
+          "name": "foo"
+        },
+        "generator": false,
+        "async": false,
+        "params": [],
+        "body": {
+          "type": "BlockStatement",
+          "start":23,"end":25,"loc":{"start":{"line":1,"column":23},"end":{"line":1,"column":25}},
+          "body": [],
+          "directives": []
+        }
+      }
+    ],
+    "directives": []
+  }
+}
\ No newline at end of file
diff --git a/packages/babel-parser/test/fixtures/typescript/declare/module/input.ts b/packages/babel-parser/test/fixtures/typescript/declare/module/input.ts
new file mode 100644
index 0000000000..fca55e911f
--- /dev/null
+++ b/packages/babel-parser/test/fixtures/typescript/declare/module/input.ts
@@ -0,0 +1,3 @@
+declare module m {
+  function foo() {}
+}
diff --git a/packages/babel-parser/test/fixtures/typescript/declare/module/output.json b/packages/babel-parser/test/fixtures/typescript/declare/module/output.json
new file mode 100644
index 0000000000..ce844216f0
--- /dev/null
+++ b/packages/babel-parser/test/fixtures/typescript/declare/module/output.json
@@ -0,0 +1,50 @@
+{
+  "type": "File",
+  "start":0,"end":40,"loc":{"start":{"line":1,"column":0},"end":{"line":3,"column":1}},
+  "errors": [
+    "SyntaxError: An implementation cannot be declared in ambient contexts. (2:2)"
+  ],
+  "program": {
+    "type": "Program",
+    "start":0,"end":40,"loc":{"start":{"line":1,"column":0},"end":{"line":3,"column":1}},
+    "sourceType": "module",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "TSModuleDeclaration",
+        "start":0,"end":40,"loc":{"start":{"line":1,"column":0},"end":{"line":3,"column":1}},
+        "id": {
+          "type": "Identifier",
+          "start":15,"end":16,"loc":{"start":{"line":1,"column":15},"end":{"line":1,"column":16},"identifierName":"m"},
+          "name": "m"
+        },
+        "body": {
+          "type": "TSModuleBlock",
+          "start":17,"end":40,"loc":{"start":{"line":1,"column":17},"end":{"line":3,"column":1}},
+          "body": [
+            {
+              "type": "FunctionDeclaration",
+              "start":21,"end":38,"loc":{"start":{"line":2,"column":2},"end":{"line":2,"column":19}},
+              "id": {
+                "type": "Identifier",
+                "start":30,"end":33,"loc":{"start":{"line":2,"column":11},"end":{"line":2,"column":14},"identifierName":"foo"},
+                "name": "foo"
+              },
+              "generator": false,
+              "async": false,
+              "params": [],
+              "body": {
+                "type": "BlockStatement",
+                "start":36,"end":38,"loc":{"start":{"line":2,"column":17},"end":{"line":2,"column":19}},
+                "body": [],
+                "directives": []
+              }
+            }
+          ]
+        },
+        "declare": true
+      }
+    ],
+    "directives": []
+  }
+}
\ No newline at end of file
diff --git a/packages/babel-parser/test/fixtures/typescript/declare/namespace/input.ts b/packages/babel-parser/test/fixtures/typescript/declare/namespace/input.ts
new file mode 100644
index 0000000000..c7f3afddc9
--- /dev/null
+++ b/packages/babel-parser/test/fixtures/typescript/declare/namespace/input.ts
@@ -0,0 +1,3 @@
+declare namespace n {
+  function foo() {}
+}
diff --git a/packages/babel-parser/test/fixtures/typescript/declare/namespace/output.json b/packages/babel-parser/test/fixtures/typescript/declare/namespace/output.json
new file mode 100644
index 0000000000..251434a134
--- /dev/null
+++ b/packages/babel-parser/test/fixtures/typescript/declare/namespace/output.json
@@ -0,0 +1,50 @@
+{
+  "type": "File",
+  "start":0,"end":43,"loc":{"start":{"line":1,"column":0},"end":{"line":3,"column":1}},
+  "errors": [
+    "SyntaxError: An implementation cannot be declared in ambient contexts. (2:2)"
+  ],
+  "program": {
+    "type": "Program",
+    "start":0,"end":43,"loc":{"start":{"line":1,"column":0},"end":{"line":3,"column":1}},
+    "sourceType": "module",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "TSModuleDeclaration",
+        "start":0,"end":43,"loc":{"start":{"line":1,"column":0},"end":{"line":3,"column":1}},
+        "id": {
+          "type": "Identifier",
+          "start":18,"end":19,"loc":{"start":{"line":1,"column":18},"end":{"line":1,"column":19},"identifierName":"n"},
+          "name": "n"
+        },
+        "body": {
+          "type": "TSModuleBlock",
+          "start":20,"end":43,"loc":{"start":{"line":1,"column":20},"end":{"line":3,"column":1}},
+          "body": [
+            {
+              "type": "FunctionDeclaration",
+              "start":24,"end":41,"loc":{"start":{"line":2,"column":2},"end":{"line":2,"column":19}},
+              "id": {
+                "type": "Identifier",
+                "start":33,"end":36,"loc":{"start":{"line":2,"column":11},"end":{"line":2,"column":14},"identifierName":"foo"},
+                "name": "foo"
+              },
+              "generator": false,
+              "async": false,
+              "params": [],
+              "body": {
+                "type": "BlockStatement",
+                "start":39,"end":41,"loc":{"start":{"line":2,"column":17},"end":{"line":2,"column":19}},
+                "body": [],
+                "directives": []
+              }
+            }
+          ]
+        },
+        "declare": true
+      }
+    ],
+    "directives": []
+  }
+}
\ No newline at end of file