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 5d23046ec7726..9bcc749c23dce 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
@@ -1540,7 +1540,7 @@ impl MaterializingSortMergeJoinStream {
     /// gathers columns across sources. A null-row sentinel at source index 0
     /// handles null right indices (unmatched streamed rows).
     fn materialize_right_columns(
-        &self,
+        &mut self,
         matched_chunks: &[(usize, UInt64Array, UInt64Array)],
         total_matched_rows: usize,
     ) -> Result<Vec<ArrayRef>> {
@@ -1555,6 +1555,19 @@ impl MaterializingSortMergeJoinStream {
                     matched_chunks.iter().map(|c| &c.2 as &dyn Array).collect();
                 as_uint64_array(&compute::concat(&refs)?)?.clone()
             };
+
+            let spill_reservation = self.reservation.new_empty();
+            if matches!(
+                &self.buffered_data.batches[first_batch_idx].batch,
+                BufferedBatchState::Spilled(_)
+            ) {
+                spill_reservation
+                    .grow(self.buffered_data.batches[first_batch_idx].size_estimation);
+                self.join_metrics
+                    .peak_mem_used()
+                    .set_max(self.reservation.size() + spill_reservation.size());
+            }
+
             return fetch_right_columns_by_idxs(
                 &self.buffered_data,
                 first_batch_idx,
@@ -1588,24 +1601,33 @@ impl MaterializingSortMergeJoinStream {
         }
 
         let num_right_cols = self.buffered_schema.fields().len();
-        let mut right_columns = Vec::with_capacity(num_right_cols);
 
         // Read each source batch once (spilled batches require disk I/O).
-        let source_data: Vec<Option<RecordBatch>> = source_batches
-            .iter()
-            .map(|&idx| {
-                let bb = &self.buffered_data.batches[idx];
-                match &bb.batch {
-                    BufferedBatchState::InMemory(batch) => Some(batch.clone()),
-                    BufferedBatchState::Spilled(spill_file) => {
-                        let file = BufReader::new(File::open(spill_file.path()).ok()?);
-                        let reader = StreamReader::try_new(file, None).ok()?;
-                        reader.into_iter().next()?.ok()
-                    }
+        // Track memory for each spilled batch at the point of deserialization
+        // so the pool reflects actual usage as it grows.
+        let spill_reservation = self.reservation.new_empty();
+        let mut source_data: Vec<Option<RecordBatch>> =
+            Vec::with_capacity(source_batches.len());
+        for &idx in &source_batches {
+            let bb = &self.buffered_data.batches[idx];
+            match &bb.batch {
+                BufferedBatchState::InMemory(batch) => {
+                    source_data.push(Some(batch.clone()));
                 }
-            })
-            .collect();
+                BufferedBatchState::Spilled(spill_file) => {
+                    spill_reservation.grow(bb.size_estimation);
+                    self.join_metrics
+                        .peak_mem_used()
+                        .set_max(self.reservation.size() + spill_reservation.size());
+
+                    let file = BufReader::new(File::open(spill_file.path())?);
+                    let reader = StreamReader::try_new(file, None)?;
+                    source_data.push(reader.into_iter().next().transpose()?);
+                }
+            }
+        }
 
+        let mut right_columns = Vec::with_capacity(num_right_cols);
         for col_idx in 0..num_right_cols {
             let dtype = self.buffered_schema.field(col_idx).data_type();
             let null_array = new_null_array(dtype, 1);
@@ -1624,7 +1646,6 @@ impl MaterializingSortMergeJoinStream {
                     }
                 }
             }
-
             right_columns.push(interleave(&source_arrays, &interleave_indices)?);
         }
 
diff --git a/datafusion/physical-plan/src/joins/sort_merge_join/tests.rs b/datafusion/physical-plan/src/joins/sort_merge_join/tests.rs
index bc34c351c5e21..c4377b3189ff7 100644
--- a/datafusion/physical-plan/src/joins/sort_merge_join/tests.rs
+++ b/datafusion/physical-plan/src/joins/sort_merge_join/tests.rs
@@ -4724,3 +4724,226 @@ async fn spill_filtered_boundary_loses_outer_rows() -> Result<()> {
 
     Ok(())
 }
+
+/// Verifies that `peak_mem_used` reflects spill read-back memory during
+/// output materialization (multi-source path).
+///
+/// When spilled buffered batches are read back from disk to produce join
+/// output, a scoped `MemoryReservation` (via `new_empty()`) tracks the
+/// transient memory. Its `Drop` guarantees the pool is balanced on every
+/// exit path — normal return or early `?` error.
+#[tokio::test]
+async fn spill_read_back_memory_accounting() -> Result<()> {
+    use arrow::array::Array;
+
+    let left_batch = build_table_i32(
+        ("a1", &vec![0, 1]),
+        ("b1", &vec![1, 1]),
+        ("c1", &vec![4, 5]),
+    );
+    let size_estimation = left_batch.get_array_memory_size()
+        + Int32Array::from(vec![1, 1]).get_array_memory_size()
+        + 2usize.next_power_of_two() * size_of::<usize>()
+        + size_of::<std::ops::Range<usize>>()
+        + size_of::<usize>();
+
+    // Memory limit too small for a full batch — forces spilling.
+    let memory_limit = size_estimation / 2;
+
+    // All rows share the same join key (b=1) to force multiple buffered
+    // batches in the same key group — triggering spill read-back during
+    // output materialization.
+    let left_batches: Vec<RecordBatch> = (0..4)
+        .map(|i| {
+            build_table_i32(
+                ("a1", &vec![i * 2, i * 2 + 1]),
+                ("b1", &vec![1, 1]),
+                ("c1", &vec![100 + i, 101 + i]),
+            )
+        })
+        .collect();
+    let left = build_table_from_batches(left_batches);
+
+    let right_batches: Vec<RecordBatch> = (0..4)
+        .map(|i| {
+            build_table_i32(
+                ("a2", &vec![i * 2, i * 2 + 1]),
+                ("b2", &vec![1, 1]),
+                ("c2", &vec![200 + i, 201 + i]),
+            )
+        })
+        .collect();
+    let right = build_table_from_batches(right_batches);
+
+    let on = vec![(
+        Arc::new(Column::new_with_schema("b1", &left.schema())?) as _,
+        Arc::new(Column::new_with_schema("b2", &right.schema())?) as _,
+    )];
+    let sort_options = vec![SortOptions::default(); on.len()];
+
+    let runtime = RuntimeEnvBuilder::new()
+        .with_memory_limit(memory_limit, 1.0)
+        .with_disk_manager_builder(
+            DiskManagerBuilder::default().with_mode(DiskManagerMode::OsTmpDirectory),
+        )
+        .build_arc()?;
+
+    let session_config = SessionConfig::default().with_batch_size(50);
+    let task_ctx = Arc::new(
+        TaskContext::default()
+            .with_session_config(session_config)
+            .with_runtime(Arc::clone(&runtime)),
+    );
+
+    let join = join_with_options(
+        Arc::clone(&left),
+        Arc::clone(&right),
+        on.clone(),
+        Inner,
+        sort_options,
+        NullEquality::NullEqualsNothing,
+    )?;
+
+    let stream = join.execute(0, task_ctx)?;
+    let result = common::collect(stream).await.unwrap();
+
+    assert!(!result.is_empty(), "Expected non-empty join result");
+
+    let metrics = join.metrics().unwrap();
+    assert!(
+        metrics.spill_count().unwrap() > 0,
+        "Expected spilling to occur"
+    );
+
+    // peak_mem_used should reflect the spill read-back: when buffered
+    // batches are read from disk during output materialization, grow()
+    // temporarily reserves size_estimation. This pushes peak above what
+    // join_arrays_mem alone would show.
+    let peak_mem = metrics
+        .sum_by_name("peak_mem_used")
+        .map(|m| m.as_usize())
+        .unwrap_or(0);
+    assert!(
+        peak_mem >= size_estimation,
+        "peak_mem_used ({peak_mem}) should be >= size_estimation ({size_estimation}) \
+         because spill read-back temporarily loads full batch into memory"
+    );
+
+    // All memory must be released (grow/shrink balanced)
+    assert_eq!(
+        runtime.memory_pool.reserved(),
+        0,
+        "All memory should be released after join completes"
+    );
+
+    Ok(())
+}
+
+/// Verifies spill read-back memory tracking for the single-source path.
+///
+/// When only ONE buffered batch exists for a key group and it's spilled,
+/// `fetch_right_columns_by_idxs` reads it back. A scoped `MemoryReservation`
+/// (via `new_empty()`) tracks the transient memory and releases it on drop.
+#[tokio::test]
+async fn spill_read_back_single_source() -> Result<()> {
+    use arrow::array::Array;
+
+    let left_batch = build_table_i32(
+        ("a1", &vec![0, 1]),
+        ("b1", &vec![1, 1]),
+        ("c1", &vec![4, 5]),
+    );
+    let size_estimation = left_batch.get_array_memory_size()
+        + Int32Array::from(vec![1, 1]).get_array_memory_size()
+        + 2usize.next_power_of_two() * size_of::<usize>()
+        + size_of::<std::ops::Range<usize>>()
+        + size_of::<usize>();
+
+    // Memory limit too small for a full batch — forces spilling.
+    let memory_limit = size_estimation / 2;
+
+    // Multiple distinct keys so each key group has exactly ONE buffered batch.
+    // This ensures the single-source path is exercised.
+    let left_batches: Vec<RecordBatch> = (0..4)
+        .map(|i| {
+            build_table_i32(
+                ("a1", &vec![i * 2, i * 2 + 1]),
+                ("b1", &vec![i, i]),
+                ("c1", &vec![100 + i, 101 + i]),
+            )
+        })
+        .collect();
+    let left = build_table_from_batches(left_batches);
+
+    // One batch per key — each key group has single source
+    let right_batches: Vec<RecordBatch> = (0..4)
+        .map(|i| {
+            build_table_i32(
+                ("a2", &vec![i * 2, i * 2 + 1]),
+                ("b2", &vec![i, i]),
+                ("c2", &vec![200 + i, 201 + i]),
+            )
+        })
+        .collect();
+    let right = build_table_from_batches(right_batches);
+
+    let on = vec![(
+        Arc::new(Column::new_with_schema("b1", &left.schema())?) as _,
+        Arc::new(Column::new_with_schema("b2", &right.schema())?) as _,
+    )];
+    let sort_options = vec![SortOptions::default(); on.len()];
+
+    let runtime = RuntimeEnvBuilder::new()
+        .with_memory_limit(memory_limit, 1.0)
+        .with_disk_manager_builder(
+            DiskManagerBuilder::default().with_mode(DiskManagerMode::OsTmpDirectory),
+        )
+        .build_arc()?;
+
+    let session_config = SessionConfig::default().with_batch_size(50);
+    let task_ctx = Arc::new(
+        TaskContext::default()
+            .with_session_config(session_config)
+            .with_runtime(Arc::clone(&runtime)),
+    );
+
+    let join = join_with_options(
+        Arc::clone(&left),
+        Arc::clone(&right),
+        on.clone(),
+        Inner,
+        sort_options,
+        NullEquality::NullEqualsNothing,
+    )?;
+
+    let stream = join.execute(0, task_ctx)?;
+    let result = common::collect(stream).await.unwrap();
+
+    assert!(!result.is_empty(), "Expected non-empty join result");
+
+    let metrics = join.metrics().unwrap();
+    assert!(
+        metrics.spill_count().unwrap() > 0,
+        "Expected spilling to occur"
+    );
+
+    // peak_mem_used should reflect the single-batch read-back
+    let peak_mem = metrics
+        .sum_by_name("peak_mem_used")
+        .map(|m| m.as_usize())
+        .unwrap_or(0);
+    assert!(
+        peak_mem >= size_estimation,
+        "peak_mem_used ({peak_mem}) should be >= size_estimation ({size_estimation}) \
+         because single-source spill read-back loads full batch"
+    );
+
+    // All memory must be released
+    assert_eq!(
+        runtime.memory_pool.reserved(),
+        0,
+        "All memory should be released after join completes"
+    );
+
+    Ok(())
+}