diff --git a/datafusion/physical-plan/src/joins/sort_merge_join/materializing_stream.rs b/datafusion/physical-plan/src/joins/sort_merge_join/materializing_stream.rs
index c387e05390fc2..1c2a037bb5108 100644
--- a/datafusion/physical-plan/src/joins/sort_merge_join/materializing_stream.rs
+++ b/datafusion/physical-plan/src/joins/sort_merge_join/materializing_stream.rs
@@ -204,6 +204,24 @@ impl StreamedBatch {
     }
 }
 
+/// Per-row filter outcome tracking for full outer joins.
+///
+/// In a full outer join with a filter, buffered rows that match on join
+/// keys but fail every filter evaluation must be emitted with NULLs on
+/// the streamed side. Three states are needed because a simple boolean
+/// cannot distinguish "never matched" (handled by [`BufferedBatch::null_joined`])
+/// from "matched but all filters failed" (must be emitted as null-joined).
+#[repr(u8)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(super) enum FilterState {
+    /// Row never appeared in a matched pair.
+    Unvisited = 0,
+    /// Row matched streamed rows, but all filter evaluations failed.
+    AllFailed = 1,
+    /// Row matched and at least one filter evaluation passed.
+    SomePassed = 2,
+}
+
 /// A buffered batch that contains contiguous rows with same join key
 ///
 /// `BufferedBatch` can exist as either an in-memory `RecordBatch` or a `RefCountedTempFile` on disk.
@@ -219,11 +237,9 @@ pub(super) struct BufferedBatch {
     pub null_joined: Vec<usize>,
     /// Size estimation used for reserving / releasing memory
     pub size_estimation: usize,
-    /// The indices of buffered batch that the join filter doesn't satisfy.
-    /// This is a map between right row index and a boolean value indicating whether all joined row
-    /// of the right row does not satisfy the filter .
-    /// When dequeuing the buffered batch, we need to produce null joined rows for these indices.
-    pub join_filter_not_matched_map: HashMap<u64, bool>,
+    /// Tracks filter outcomes for buffered rows in full outer joins.
+    /// Indexed by absolute row position within the batch. See [`FilterState`].
+    pub join_filter_status: Vec<FilterState>,
     /// Current buffered batch number of rows. Equal to batch.num_rows()
     /// but if batch is spilled to disk this property is preferable
     /// and less expensive
@@ -260,7 +276,7 @@ impl BufferedBatch {
             join_arrays,
             null_joined: vec![],
             size_estimation,
-            join_filter_not_matched_map: HashMap::new(),
+            join_filter_status: vec![FilterState::Unvisited; num_rows],
             num_rows,
         }
     }
@@ -1199,12 +1215,16 @@ impl MaterializingSortMergeJoinStream {
             return Ok(());
         }
 
-        // For buffered row which is joined with streamed side rows but all joined rows
-        // don't satisfy the join filter
+        // Collect buffered rows that matched on join keys but had every
+        // filter evaluation fail — these must be emitted with NULLs on
+        // the streamed side to satisfy full outer join semantics.
         let not_matched_buffered_indices = buffered_batch
-            .join_filter_not_matched_map
+            .join_filter_status
             .iter()
-            .filter_map(|(idx, failed)| if *failed { Some(*idx) } else { None })
+            .enumerate()
+            .filter_map(|(i, state)| {
+                matches!(state, FilterState::AllFailed).then_some(i as u64)
+            })
             .collect::<Vec<_>>();
 
         let buffered_indices =
@@ -1219,7 +1239,9 @@ impl MaterializingSortMergeJoinStream {
             self.joined_record_batches
                 .push_batch_with_null_metadata(record_batch, self.join_type);
         }
-        buffered_batch.join_filter_not_matched_map.clear();
+        buffered_batch
+            .join_filter_status
+            .fill(FilterState::Unvisited);
 
         Ok(())
     }
@@ -1392,15 +1414,18 @@ impl MaterializingSortMergeJoinStream {
                             if right.is_null(i) {
                                 continue;
                             }
-                            let buffered_index = right.value(i);
-                            buffered_batch.join_filter_not_matched_map.insert(
-                                buffered_index,
-                                *buffered_batch
-                                    .join_filter_not_matched_map
-                                    .get(&buffered_index)
-                                    .unwrap_or(&true)
-                                    && !pre_mask.value(offset + i),
-                            );
+                            let idx = right.value(i) as usize;
+                            match buffered_batch.join_filter_status[idx] {
+                                FilterState::SomePassed => {}
+                                _ if pre_mask.value(offset + i) => {
+                                    buffered_batch.join_filter_status[idx] =
+                                        FilterState::SomePassed;
+                                }
+                                _ => {
+                                    buffered_batch.join_filter_status[idx] =
+                                        FilterState::AllFailed;
+                                }
+                            }
                         }
                         offset += chunk_len;
                     }