diff --git a/.changeset/fix-left-join-where-filter.md b/.changeset/fix-left-join-where-filter.md
new file mode 100644
index 000000000..1a03f0797
--- /dev/null
+++ b/.changeset/fix-left-join-where-filter.md
@@ -0,0 +1,7 @@
+---
+'@tanstack/db': patch
+---
+
+fix(db): don't push WHERE clauses to nullable side of outer joins
+
+The query optimizer incorrectly pushed single-source WHERE clauses into subqueries and collection index optimization for the nullable side of outer joins. This pre-filtered the data before the join, converting rows that should have been excluded by the WHERE into unmatched outer-join rows that incorrectly survived the residual filter.
diff --git a/packages/db/src/query/optimizer.ts b/packages/db/src/query/optimizer.ts
index e1020c284..e962f296f 100644
--- a/packages/db/src/query/optimizer.ts
+++ b/packages/db/src/query/optimizer.ts
@@ -219,8 +219,11 @@ export function optimizeQuery(query: QueryIR): OptimizationResult {
 
 /**
  * Extracts collection-specific WHERE clauses from a query for index optimization.
- * This analyzes the original query to identify WHERE clauses that can be pushed down
- * to specific collections, but only for simple queries without joins.
+ * This analyzes the original query to identify single-source WHERE clauses that
+ * reference collection sources (not subqueries), including joined collections.
+ *
+ * For outer joins, clauses referencing the nullable side are excluded because
+ * using them to pre-filter collection data would change join semantics.
  *
  * @param query - The original QueryIR to analyze
  * @returns Map of source aliases to their WHERE clauses
@@ -246,10 +249,19 @@ function extractSourceWhereClauses(
   // Group clauses by single-source vs multi-source
   const groupedClauses = groupWhereClauses(analyzedClauses)
 
+  // Determine which source aliases are on the nullable side of outer joins.
+  // WHERE clauses for these sources must not be used for index optimization
+  // because they should filter the final joined result, not the input data.
+  const nullableSources = getNullableJoinSources(query)
+
   // Only include single-source clauses that reference collections directly
+  // and are not on the nullable side of an outer join
   for (const [sourceAlias, whereClause] of groupedClauses.singleSource) {
     // Check if this source alias corresponds to a collection reference
-    if (isCollectionReference(query, sourceAlias)) {
+    if (
+      isCollectionReference(query, sourceAlias) &&
+      !nullableSources.has(sourceAlias)
+    ) {
       sourceWhereClauses.set(sourceAlias, whereClause)
     }
   }
@@ -283,6 +295,36 @@ function isCollectionReference(query: QueryIR, sourceAlias: string): boolean {
   return false
 }
 
+/**
+ * Returns the set of source aliases that are on the nullable side of outer joins.
+ *
+ * For a LEFT join the joined (right) side is nullable.
+ * For a RIGHT join the main (left/from) side is nullable.
+ * For a FULL join both sides are nullable.
+ *
+ * WHERE clauses that reference only a nullable source must not be pushed down
+ * into that source's subquery or used for index optimization, because doing so
+ * changes the join semantics: rows that should be excluded by the WHERE become
+ * unmatched outer-join rows (with the nullable side set to undefined) and
+ * incorrectly survive residual filtering.
+ */
+function getNullableJoinSources(query: QueryIR): Set<string> {
+  const nullable = new Set<string>()
+  if (query.join) {
+    const mainAlias = query.from.alias
+    for (const join of query.join) {
+      const joinedAlias = join.from.alias
+      if (join.type === `left` || join.type === `full`) {
+        nullable.add(joinedAlias)
+      }
+      if (join.type === `right` || join.type === `full`) {
+        nullable.add(mainAlias)
+      }
+    }
+  }
+  return nullable
+}
+
 /**
  * Applies recursive predicate pushdown optimization.
  *
@@ -635,10 +677,25 @@ function applyOptimizations(
   // Track which single-source clauses were actually optimized
   const actuallyOptimized = new Set<string>()
 
+  // Determine which source aliases are on the nullable side of outer joins.
+  const nullableSources = getNullableJoinSources(query)
+
+  // Build a filtered copy of singleSource that excludes nullable-side clauses.
+  // Pushing a WHERE clause into the nullable side's subquery pre-filters the
+  // data before the join, converting "matched but WHERE-excluded" rows into
+  // "unmatched" outer-join rows.  These are indistinguishable from genuinely
+  // unmatched rows, so the residual WHERE cannot correct the result.
+  const pushableSingleSource = new Map<string, BasicExpression<boolean>>()
+  for (const [source, clause] of groupedClauses.singleSource) {
+    if (!nullableSources.has(source)) {
+      pushableSingleSource.set(source, clause)
+    }
+  }
+
   // Optimize the main FROM clause and track what was optimized
   const optimizedFrom = optimizeFromWithTracking(
     query.from,
-    groupedClauses.singleSource,
+    pushableSingleSource,
     actuallyOptimized,
   )
 
@@ -648,7 +705,7 @@ function applyOptimizations(
         ...joinClause,
         from: optimizeFromWithTracking(
           joinClause.from,
-          groupedClauses.singleSource,
+          pushableSingleSource,
           actuallyOptimized,
         ),
       }))
@@ -663,12 +720,7 @@ function applyOptimizations(
   }
 
   // Determine if we need residual clauses (when query has outer JOINs)
-  const hasOuterJoins =
-    query.join &&
-    query.join.some(
-      (join) =>
-        join.type === `left` || join.type === `right` || join.type === `full`,
-    )
+  const hasOuterJoins = nullableSources.size > 0
 
   // Add single-source clauses
   for (const [source, clause] of groupedClauses.singleSource) {
diff --git a/packages/db/tests/query/indexes.test.ts b/packages/db/tests/query/indexes.test.ts
index 9b221443c..a826861aa 100644
--- a/packages/db/tests/query/indexes.test.ts
+++ b/packages/db/tests/query/indexes.test.ts
@@ -679,13 +679,14 @@ describe(`Query Index Optimization`, () => {
           ],
         }
 
-        // Should use index optimization for both WHERE clauses
-        // since they each touch only a single source and both sources are indexed
+        // The WHERE clause on the non-nullable (left) side uses its index.
+        // The WHERE clause on the nullable (right) side of the LEFT JOIN is NOT
+        // pushed down to avoid changing join semantics, so the right side does a full scan.
         expectIndexUsage(combinedStats, {
           shouldUseIndex: true,
-          shouldUseFullScan: false,
-          indexCallCount: 2, // Both item.status='active' and other.status='active' can use indexes
-          fullScanCallCount: 0,
+          shouldUseFullScan: true,
+          indexCallCount: 1, // Only item.status='active' uses index (non-nullable side)
+          fullScanCallCount: 1, // other collection does full scan (nullable side)
         })
       } finally {
         tracker1.restore()
@@ -1177,21 +1178,18 @@ describe(`Query Index Optimization`, () => {
           { id: `1`, name: `Alice`, otherName: `Other Active Item` },
         ])
 
-        // We should have done a full scan of the right collection
+        // The right collection does a full scan (no index on status)
         expect(tracker2.stats.queriesExecuted).toEqual([
           {
             type: `fullScan`,
           },
         ])
 
-        // We should have done an index lookup on the 1st collection to find active items
+        // In a RIGHT join, the left (from) side is nullable. The WHERE clause
+        // eq(item.status, 'active') is NOT pushed down to avoid changing join
+        // semantics, so the left collection does NOT do an index lookup for status.
+        // It only does the index lookup for the join key (id) used by lazy loading.
         expect(tracker1.stats.queriesExecuted).toEqual([
-          {
-            field: `status`,
-            operation: `eq`,
-            type: `index`,
-            value: `active`,
-          },
           {
             type: `index`,
             operation: `in`,
@@ -1281,14 +1279,12 @@ describe(`Query Index Optimization`, () => {
           },
         ])
 
-        // We should have done an index lookup on the left collection to find active items
-        // because it has an index on the join key
+        // In a RIGHT join, the left (from) side is nullable. The WHERE clause
+        // eq(item.status, 'active') is NOT pushed down to avoid changing join
+        // semantics, so the left collection does a full scan.
         expect(tracker1.stats.queriesExecuted).toEqual([
           {
-            type: `index`,
-            operation: `eq`,
-            field: `status`,
-            value: `active`,
+            type: `fullScan`,
           },
         ])
       } finally {
diff --git a/packages/db/tests/query/join.test.ts b/packages/db/tests/query/join.test.ts
index d0918564e..441be59bc 100644
--- a/packages/db/tests/query/join.test.ts
+++ b/packages/db/tests/query/join.test.ts
@@ -1840,6 +1840,66 @@ function createJoinTests(autoIndex: `off` | `eager`): void {
     expect(result.join2!.value).toBe(1)
     expect(result.join2!.other).toBe(30)
   })
+
+  test(`where with isUndefined on right-side field should filter entire joined rows`, () => {
+    type Left = {
+      id: string
+      rightId: string | null
+    }
+
+    type Right = {
+      id: string
+      payload: string | null | undefined
+    }
+
+    const leftCollection = createCollection(
+      mockSyncCollectionOptions<Left>({
+        id: `test-left-isundefined-field`,
+        getKey: (item) => item.id,
+        initialData: [
+          { id: `l1`, rightId: `r1` },
+          { id: `l2`, rightId: `r2` },
+          { id: `l3`, rightId: `r3` },
+          { id: `l4`, rightId: null },
+        ],
+        autoIndex,
+      }),
+    )
+
+    const rightCollection = createCollection(
+      mockSyncCollectionOptions<Right>({
+        id: `test-right-isundefined-field`,
+        getKey: (item) => item.id,
+        initialData: [
+          { id: `r1`, payload: `ok` },
+          { id: `r2`, payload: null },
+          { id: `r3`, payload: undefined },
+        ],
+        autoIndex,
+      }),
+    )
+
+    const lq = createLiveQueryCollection({
+      startSync: true,
+      query: (q) =>
+        q
+          .from({ l: leftCollection })
+          .leftJoin({ r: rightCollection }, ({ l, r }) => eq(l.rightId, r.id))
+          .where(({ r }) => isUndefined(r?.payload))
+          .select(({ l, r }) => ({ leftId: l.id, right: r })),
+    })
+
+    const data = lq.toArray
+
+    // l1 joins r1 (payload='ok') → payload is defined → exclude
+    // l2 joins r2 (payload=null) → payload is null, not undefined → exclude
+    // l3 joins r3 (payload=undefined) → payload is undefined → include
+    // l4 has no match (rightId=null) → right is undefined, so r?.payload is undefined → include
+    expect(data.sort((a, b) => a.leftId.localeCompare(b.leftId))).toEqual([
+      { leftId: `l3`, right: { id: `r3`, payload: undefined } },
+      { leftId: `l4`, right: undefined },
+    ])
+  })
 }
 
 describe(`Query JOIN Operations`, () => {
diff --git a/packages/db/tests/query/optimizer.test.ts b/packages/db/tests/query/optimizer.test.ts
index 77ff8cbed..0952ac2ba 100644
--- a/packages/db/tests/query/optimizer.test.ts
+++ b/packages/db/tests/query/optimizer.test.ts
@@ -1649,35 +1649,15 @@ describe(`Query Optimizer`, () => {
       const { optimizedQuery } = optimizeQuery(query)
 
       // The WHERE clause should remain in the main query to preserve LEFT JOIN semantics
-      // It should NOT be completely moved to the subquery
+      // It must NOT be pushed down to the nullable (right) side because that would
+      // pre-filter the right data, converting excluded matches into unmatched rows
       expect(optimizedQuery.where).toHaveLength(1)
-      expect(optimizedQuery.where![0]).toEqual({
-        expression: createEq(
-          createPropRef(`teamMember`, `user_id`),
-          createValue(100),
-        ),
-        residual: true,
-      })
+      expect(optimizedQuery.where![0]).toEqual(
+        createEq(createPropRef(`teamMember`, `user_id`), createValue(100)),
+      )
 
-      // If the optimizer creates a subquery for teamMember, the WHERE clause should also be copied there
-      // but a residual copy must remain in the main query
-      if (
-        optimizedQuery.join &&
-        optimizedQuery.join[0]?.from.type === `queryRef`
-      ) {
-        const teamMemberSubquery = optimizedQuery.join[0].from.query
-        // The subquery may have the WHERE clause for optimization
-        if (teamMemberSubquery.where && teamMemberSubquery.where.length > 0) {
-          // But the main query MUST still have it to preserve semantics
-          expect(optimizedQuery.where).toContainEqual({
-            expression: createEq(
-              createPropRef(`teamMember`, `user_id`),
-              createValue(100),
-            ),
-            residual: true,
-          })
-        }
-      }
+      // The optimizer should NOT create a subquery for the nullable side
+      expect(optimizedQuery.join![0]!.from.type).toBe(`collectionRef`)
     })
 
     test(`should preserve WHERE clause semantics when pushing down to RIGHT JOIN`, () => {
@@ -1715,32 +1695,14 @@ describe(`Query Optimizer`, () => {
       const { optimizedQuery } = optimizeQuery(query)
 
       // The WHERE clause should remain in the main query to preserve RIGHT JOIN semantics
-      // It should NOT be completely moved to the subquery
+      // It must NOT be pushed down to the nullable (left/from) side
       expect(optimizedQuery.where).toHaveLength(1)
-      expect(optimizedQuery.where![0]).toEqual({
-        expression: createEq(
-          createPropRef(`user`, `department_id`),
-          createValue(1),
-        ),
-        residual: true,
-      })
+      expect(optimizedQuery.where![0]).toEqual(
+        createEq(createPropRef(`user`, `department_id`), createValue(1)),
+      )
 
-      // If the optimizer creates a subquery for users, the WHERE clause should also be copied there
-      // but a residual copy must remain in the main query
-      if (optimizedQuery.from.type === `queryRef`) {
-        const userSubquery = optimizedQuery.from.query
-        // The subquery may have the WHERE clause for optimization
-        if (userSubquery.where && userSubquery.where.length > 0) {
-          // But the main query MUST still have it to preserve semantics
-          expect(optimizedQuery.where).toContainEqual({
-            expression: createEq(
-              createPropRef(`user`, `department_id`),
-              createValue(1),
-            ),
-            residual: true,
-          })
-        }
-      }
+      // The optimizer should NOT create a subquery for the nullable side (from)
+      expect(optimizedQuery.from.type).toBe(`collectionRef`)
     })
 
     test(`should preserve WHERE clause semantics when pushing down to FULL JOIN`, () => {
@@ -1778,35 +1740,15 @@ describe(`Query Optimizer`, () => {
       const { optimizedQuery } = optimizeQuery(query)
 
       // The WHERE clause should remain in the main query to preserve FULL JOIN semantics
-      // It should NOT be completely moved to the subquery
+      // It must NOT be pushed down to any nullable side
       expect(optimizedQuery.where).toHaveLength(1)
-      expect(optimizedQuery.where![0]).toEqual({
-        expression: createGt(
-          createPropRef(`payment`, `amount`),
-          createValue(100),
-        ),
-        residual: true,
-      })
+      expect(optimizedQuery.where![0]).toEqual(
+        createGt(createPropRef(`payment`, `amount`), createValue(100)),
+      )
 
-      // If the optimizer creates a subquery for payments, the WHERE clause should also be copied there
-      // but a residual copy must remain in the main query
-      if (
-        optimizedQuery.join &&
-        optimizedQuery.join[0]?.from.type === `queryRef`
-      ) {
-        const paymentSubquery = optimizedQuery.join[0].from.query
-        // The subquery may have the WHERE clause for optimization
-        if (paymentSubquery.where && paymentSubquery.where.length > 0) {
-          // But the main query MUST still have it to preserve semantics
-          expect(optimizedQuery.where).toContainEqual({
-            expression: createGt(
-              createPropRef(`payment`, `amount`),
-              createValue(100),
-            ),
-            residual: true,
-          })
-        }
-      }
+      // The optimizer should NOT create subqueries for nullable sides
+      expect(optimizedQuery.from.type).toBe(`collectionRef`)
+      expect(optimizedQuery.join![0]!.from.type).toBe(`collectionRef`)
     })
 
     test(`should allow WHERE clause pushdown for INNER JOIN (semantics preserved)`, () => {