diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 2341a0f7dfd..8c1e401912d 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -18763,7 +18763,13 @@ namespace ts {
                     (augmentations || (augmentations = [])).push(file.moduleAugmentations);
                 }
                 if (file.symbol && file.symbol.globalExports) {
-                    mergeSymbolTable(globals, file.symbol.globalExports);
+                    // Merge in UMD exports with first-in-wins semantics (see #9771)
+                    const source = file.symbol.globalExports;
+                    for (const id in source) {
+                        if (!(id in globals)) {
+                            globals[id] = source[id];
+                        }
+                    }
                 }
             });
 
diff --git a/tests/baselines/reference/umdGlobalConflict.js b/tests/baselines/reference/umdGlobalConflict.js
new file mode 100644
index 00000000000..e8a820e2333
--- /dev/null
+++ b/tests/baselines/reference/umdGlobalConflict.js
@@ -0,0 +1,23 @@
+//// [tests/cases/compiler/umdGlobalConflict.ts] ////
+
+//// [index.d.ts]
+export as namespace Alpha;
+export var x: string;
+
+//// [index.d.ts]
+export as namespace Alpha;
+export var y: number;
+
+//// [consumer.ts]
+import * as v1 from './v1';
+import * as v2 from './v2';
+
+//// [global.ts]
+// Should be OK, first in wins
+const p: string = Alpha.x;
+
+//// [consumer.js]
+"use strict";
+//// [global.js]
+// Should be OK, first in wins
+var p = Alpha.x;
diff --git a/tests/baselines/reference/umdGlobalConflict.symbols b/tests/baselines/reference/umdGlobalConflict.symbols
new file mode 100644
index 00000000000..b13fb0f25b1
--- /dev/null
+++ b/tests/baselines/reference/umdGlobalConflict.symbols
@@ -0,0 +1,29 @@
+=== tests/cases/compiler/v1/index.d.ts ===
+export as namespace Alpha;
+>Alpha : Symbol(Alpha, Decl(index.d.ts, 0, 0))
+
+export var x: string;
+>x : Symbol(x, Decl(index.d.ts, 1, 10))
+
+=== tests/cases/compiler/v2/index.d.ts ===
+export as namespace Alpha;
+>Alpha : Symbol(Alpha, Decl(index.d.ts, 0, 0))
+
+export var y: number;
+>y : Symbol(y, Decl(index.d.ts, 1, 10))
+
+=== tests/cases/compiler/consumer.ts ===
+import * as v1 from './v1';
+>v1 : Symbol(v1, Decl(consumer.ts, 0, 6))
+
+import * as v2 from './v2';
+>v2 : Symbol(v2, Decl(consumer.ts, 1, 6))
+
+=== tests/cases/compiler/global.ts ===
+// Should be OK, first in wins
+const p: string = Alpha.x;
+>p : Symbol(p, Decl(global.ts, 1, 5))
+>Alpha.x : Symbol(Alpha.x, Decl(index.d.ts, 1, 10))
+>Alpha : Symbol(Alpha, Decl(index.d.ts, 0, 0))
+>x : Symbol(Alpha.x, Decl(index.d.ts, 1, 10))
+
diff --git a/tests/baselines/reference/umdGlobalConflict.types b/tests/baselines/reference/umdGlobalConflict.types
new file mode 100644
index 00000000000..fa74e26874f
--- /dev/null
+++ b/tests/baselines/reference/umdGlobalConflict.types
@@ -0,0 +1,29 @@
+=== tests/cases/compiler/v1/index.d.ts ===
+export as namespace Alpha;
+>Alpha : typeof Alpha
+
+export var x: string;
+>x : string
+
+=== tests/cases/compiler/v2/index.d.ts ===
+export as namespace Alpha;
+>Alpha : typeof 
+
+export var y: number;
+>y : number
+
+=== tests/cases/compiler/consumer.ts ===
+import * as v1 from './v1';
+>v1 : typeof v1
+
+import * as v2 from './v2';
+>v2 : typeof v2
+
+=== tests/cases/compiler/global.ts ===
+// Should be OK, first in wins
+const p: string = Alpha.x;
+>p : string
+>Alpha.x : string
+>Alpha : typeof Alpha
+>x : string
+
diff --git a/tests/cases/compiler/umdGlobalConflict.ts b/tests/cases/compiler/umdGlobalConflict.ts
new file mode 100644
index 00000000000..58b4ea6be02
--- /dev/null
+++ b/tests/cases/compiler/umdGlobalConflict.ts
@@ -0,0 +1,15 @@
+//@filename: v1/index.d.ts
+export as namespace Alpha;
+export var x: string;
+
+//@filename: v2/index.d.ts
+export as namespace Alpha;
+export var y: number;
+
+//@filename: consumer.ts
+import * as v1 from './v1';
+import * as v2 from './v2';
+
+//@filename: global.ts
+// Should be OK, first in wins
+const p: string = Alpha.x;
\ No newline at end of file