diff --git a/.changeset/silent-cars-burn.md b/.changeset/silent-cars-burn.md
new file mode 100644
index 00000000000..a526ac37e8f
--- /dev/null
+++ b/.changeset/silent-cars-burn.md
@@ -0,0 +1,5 @@
+---
+'@firebase/firestore': patch
+---
+
+handle `ignoreUndefinedProperties` in `set({ merge: true })`. Previously this would behave as if the undefined value were `FieldValue.delete()`, which wasn't intended.
diff --git a/packages/firestore/src/api/user_data_reader.ts b/packages/firestore/src/api/user_data_reader.ts
index c58da2e4209..ddb41efb927 100644
--- a/packages/firestore/src/api/user_data_reader.ts
+++ b/packages/firestore/src/api/user_data_reader.ts
@@ -738,6 +738,11 @@ export function parseData(
     // context.fieldMask and we return null as our parsing result.
     parseSentinelFieldValue(input, context);
     return null;
+  } else if (input === undefined && context.ignoreUndefinedProperties) {
+    // If the input is undefined it can never participate in the fieldMask, so
+    // don't handle this below. If `ignoreUndefinedProperties` is false,
+    // `parseScalarValue` will reject an undefined value.
+    return null;
   } else {
     // If context.path is null we are inside an array and we don't support
     // field mask paths more granular than the top-level array.
@@ -896,8 +901,6 @@ function parseScalarValue(
         value._key.path
       )
     };
-  } else if (value === undefined && context.ignoreUndefinedProperties) {
-    return null;
   } else {
     throw context.createError(
       `Unsupported field value: ${valueDescription(value)}`
diff --git a/packages/firestore/test/integration/api/fields.test.ts b/packages/firestore/test/integration/api/fields.test.ts
index a1f46f4d0c7..6d92def2493 100644
--- a/packages/firestore/test/integration/api/fields.test.ts
+++ b/packages/firestore/test/integration/api/fields.test.ts
@@ -398,6 +398,15 @@ apiDescribe('`undefined` properties', (persistence: boolean) => {
     });
   });
 
+  it('are ignored in set({ merge: true })', () => {
+    return withTestDocAndSettings(persistence, settings, async doc => {
+      await doc.set({ foo: 'foo', bar: 'unchanged' });
+      await doc.set({ foo: 'foo', bar: undefined }, { merge: true });
+      const docSnap = await doc.get();
+      expect(docSnap.data()).to.deep.equal({ foo: 'foo', bar: 'unchanged' });
+    });
+  });
+
   it('are ignored in update()', () => {
     return withTestDocAndSettings(persistence, settings, async doc => {
       await doc.set({});