diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index ddfebd84ad6..559d6ae71ba 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -9029,6 +9029,7 @@ namespace ts {
 
             function findMatchingDiscriminantType(source: Type, target: UnionOrIntersectionType) {
                 const sourceProperties = getPropertiesOfObjectType(source);
+                let match: Type;
                 if (sourceProperties) {
                     for (const sourceProperty of sourceProperties) {
                         if (isDiscriminantProperty(target, sourceProperty.name)) {
@@ -9036,12 +9037,16 @@ namespace ts {
                             for (const type of target.types) {
                                 const targetType = getTypeOfPropertyOfType(type, sourceProperty.name);
                                 if (targetType && isRelatedTo(sourceType, targetType)) {
-                                    return type;
+                                    if (match) {
+                                        return undefined;
+                                    }
+                                    match = type;
                                 }
                             }
                         }
                     }
                 }
+                return match;
             }
 
             function typeRelatedToEachType(source: Type, target: IntersectionType, reportErrors: boolean): Ternary {
diff --git a/tests/baselines/reference/excessPropertyCheckWithUnions.errors.txt b/tests/baselines/reference/excessPropertyCheckWithUnions.errors.txt
index 1a01f8cb5b8..9714c90ba61 100644
--- a/tests/baselines/reference/excessPropertyCheckWithUnions.errors.txt
+++ b/tests/baselines/reference/excessPropertyCheckWithUnions.errors.txt
@@ -5,9 +5,21 @@ tests/cases/compiler/excessPropertyCheckWithUnions.ts(11,21): error TS2322: Type
 tests/cases/compiler/excessPropertyCheckWithUnions.ts(12,1): error TS2322: Type '{ tag: "D"; }' is not assignable to type 'ADT'.
   Type '{ tag: "D"; }' is not assignable to type '{ tag: "D"; d20: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20; }'.
     Property 'd20' is missing in type '{ tag: "D"; }'.
+tests/cases/compiler/excessPropertyCheckWithUnions.ts(33,28): error TS2322: Type '{ tag: "A"; x: string; extra: number; }' is not assignable to type 'Ambiguous'.
+  Object literal may only specify known properties, and 'extra' does not exist in type 'Ambiguous'.
+tests/cases/compiler/excessPropertyCheckWithUnions.ts(34,26): error TS2322: Type '{ tag: "A"; y: number; extra: number; }' is not assignable to type 'Ambiguous'.
+  Object literal may only specify known properties, and 'extra' does not exist in type 'Ambiguous'.
+tests/cases/compiler/excessPropertyCheckWithUnions.ts(39,1): error TS2322: Type '{ tag: "A"; }' is not assignable to type 'Ambiguous'.
+  Type '{ tag: "A"; }' is not assignable to type '{ tag: "C"; }'.
+    Types of property 'tag' are incompatible.
+      Type '"A"' is not assignable to type '"C"'.
+tests/cases/compiler/excessPropertyCheckWithUnions.ts(40,1): error TS2322: Type '{ tag: "A"; z: true; }' is not assignable to type 'Ambiguous'.
+  Type '{ tag: "A"; z: true; }' is not assignable to type '{ tag: "C"; }'.
+    Types of property 'tag' are incompatible.
+      Type '"A"' is not assignable to type '"C"'.
 
 
-==== tests/cases/compiler/excessPropertyCheckWithUnions.ts (3 errors) ====
+==== tests/cases/compiler/excessPropertyCheckWithUnions.ts (7 errors) ====
     type ADT = {
         tag: "A",
         a1: string
@@ -30,4 +42,48 @@ tests/cases/compiler/excessPropertyCheckWithUnions.ts(12,1): error TS2322: Type
 !!! error TS2322: Type '{ tag: "D"; }' is not assignable to type 'ADT'.
 !!! error TS2322:   Type '{ tag: "D"; }' is not assignable to type '{ tag: "D"; d20: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20; }'.
 !!! error TS2322:     Property 'd20' is missing in type '{ tag: "D"; }'.
+    
+    type Ambiguous = {
+        tag: "A",
+        x: string
+    } | {
+        tag: "A",
+        y: number
+    } | {
+        tag: "B",
+        z: boolean
+    } | {
+        tag: "C"
+    }
+    let amb: Ambiguous
+    // no error for ambiguous tag, even when it could satisfy both constituents at once
+    amb = { tag: "A", x: "hi" }
+    amb = { tag: "A", y: 12 }
+    amb = { tag: "A", x: "hi", y: 12 }
+    
+    // correctly error on excess property 'extra', even when ambiguous
+    amb = { tag: "A", x: "hi", extra: 12 }
+                               ~~~~~~~~~
+!!! error TS2322: Type '{ tag: "A"; x: string; extra: number; }' is not assignable to type 'Ambiguous'.
+!!! error TS2322:   Object literal may only specify known properties, and 'extra' does not exist in type 'Ambiguous'.
+    amb = { tag: "A", y: 12, extra: 12 }
+                             ~~~~~~~~~
+!!! error TS2322: Type '{ tag: "A"; y: number; extra: number; }' is not assignable to type 'Ambiguous'.
+!!! error TS2322:   Object literal may only specify known properties, and 'extra' does not exist in type 'Ambiguous'.
+    
+    // assignability errors still work.
+    // But note that the error for `z: true` is the fallback one of reporting on
+    // the last constituent since assignability error reporting can't find a single best discriminant either.
+    amb = { tag: "A" }
+    ~~~
+!!! error TS2322: Type '{ tag: "A"; }' is not assignable to type 'Ambiguous'.
+!!! error TS2322:   Type '{ tag: "A"; }' is not assignable to type '{ tag: "C"; }'.
+!!! error TS2322:     Types of property 'tag' are incompatible.
+!!! error TS2322:       Type '"A"' is not assignable to type '"C"'.
+    amb = { tag: "A", z: true }
+    ~~~
+!!! error TS2322: Type '{ tag: "A"; z: true; }' is not assignable to type 'Ambiguous'.
+!!! error TS2322:   Type '{ tag: "A"; z: true; }' is not assignable to type '{ tag: "C"; }'.
+!!! error TS2322:     Types of property 'tag' are incompatible.
+!!! error TS2322:       Type '"A"' is not assignable to type '"C"'.
     
\ No newline at end of file
diff --git a/tests/baselines/reference/excessPropertyCheckWithUnions.js b/tests/baselines/reference/excessPropertyCheckWithUnions.js
index 79c30bce9c7..fb0a5a6f819 100644
--- a/tests/baselines/reference/excessPropertyCheckWithUnions.js
+++ b/tests/baselines/reference/excessPropertyCheckWithUnions.js
@@ -11,9 +11,50 @@ type ADT = {
 let wrong: ADT = { tag: "T", a1: "extra" }
 wrong = { tag: "A", d20: 12 }
 wrong = { tag: "D" }
+
+type Ambiguous = {
+    tag: "A",
+    x: string
+} | {
+    tag: "A",
+    y: number
+} | {
+    tag: "B",
+    z: boolean
+} | {
+    tag: "C"
+}
+let amb: Ambiguous
+// no error for ambiguous tag, even when it could satisfy both constituents at once
+amb = { tag: "A", x: "hi" }
+amb = { tag: "A", y: 12 }
+amb = { tag: "A", x: "hi", y: 12 }
+
+// correctly error on excess property 'extra', even when ambiguous
+amb = { tag: "A", x: "hi", extra: 12 }
+amb = { tag: "A", y: 12, extra: 12 }
+
+// assignability errors still work.
+// But note that the error for `z: true` is the fallback one of reporting on
+// the last constituent since assignability error reporting can't find a single best discriminant either.
+amb = { tag: "A" }
+amb = { tag: "A", z: true }
 
 
 //// [excessPropertyCheckWithUnions.js]
 var wrong = { tag: "T", a1: "extra" };
 wrong = { tag: "A", d20: 12 };
 wrong = { tag: "D" };
+var amb;
+// no error for ambiguous tag, even when it could satisfy both constituents at once
+amb = { tag: "A", x: "hi" };
+amb = { tag: "A", y: 12 };
+amb = { tag: "A", x: "hi", y: 12 };
+// correctly error on excess property 'extra', even when ambiguous
+amb = { tag: "A", x: "hi", extra: 12 };
+amb = { tag: "A", y: 12, extra: 12 };
+// assignability errors still work.
+// But note that the error for `z: true` is the fallback one of reporting on
+// the last constituent since assignability error reporting can't find a single best discriminant either.
+amb = { tag: "A" };
+amb = { tag: "A", z: true };
diff --git a/tests/cases/compiler/excessPropertyCheckWithUnions.ts b/tests/cases/compiler/excessPropertyCheckWithUnions.ts
index 8b1abf1b764..51f36d137fd 100644
--- a/tests/cases/compiler/excessPropertyCheckWithUnions.ts
+++ b/tests/cases/compiler/excessPropertyCheckWithUnions.ts
@@ -10,3 +10,31 @@ type ADT = {
 let wrong: ADT = { tag: "T", a1: "extra" }
 wrong = { tag: "A", d20: 12 }
 wrong = { tag: "D" }
+
+type Ambiguous = {
+    tag: "A",
+    x: string
+} | {
+    tag: "A",
+    y: number
+} | {
+    tag: "B",
+    z: boolean
+} | {
+    tag: "C"
+}
+let amb: Ambiguous
+// no error for ambiguous tag, even when it could satisfy both constituents at once
+amb = { tag: "A", x: "hi" }
+amb = { tag: "A", y: 12 }
+amb = { tag: "A", x: "hi", y: 12 }
+
+// correctly error on excess property 'extra', even when ambiguous
+amb = { tag: "A", x: "hi", extra: 12 }
+amb = { tag: "A", y: 12, extra: 12 }
+
+// assignability errors still work.
+// But note that the error for `z: true` is the fallback one of reporting on
+// the last constituent since assignability error reporting can't find a single best discriminant either.
+amb = { tag: "A" }
+amb = { tag: "A", z: true }