diff --git a/spark/v4.1/spark/src/test/java/org/apache/iceberg/spark/sql/TestStoragePartitionedJoins.java b/spark/v4.1/spark/src/test/java/org/apache/iceberg/spark/sql/TestStoragePartitionedJoins.java
index 8788dff15806..16096f283296 100644
--- a/spark/v4.1/spark/src/test/java/org/apache/iceberg/spark/sql/TestStoragePartitionedJoins.java
+++ b/spark/v4.1/spark/src/test/java/org/apache/iceberg/spark/sql/TestStoragePartitionedJoins.java
@@ -175,6 +175,87 @@ public void testJoinsWithBucketingOnDecimalColumn() throws NoSuchTableException
     checkJoin("decimal_col", "DECIMAL(20, 2)", "bucket(8, decimal_col)");
   }
 
+  @TestTemplate
+  public void testJoinWithBucketStringSubsetOfPartitionKeys() throws NoSuchTableException {
+    // Regression test for #15349: join keys that are a subset of partition keys
+    // with bucket transforms on String columns
+    String createTableStmt =
+        "CREATE TABLE %s (user_id STRING, post_id STRING, "
+            + "collected_date STRING, category_id STRING, payload STRING)"
+            + "USING iceberg "
+            + "PARTITIONED BY (collected_date, bucket(32, user_id), bucket(32, category_id))"
+            + "TBLPROPERTIES (%s)";
+
+    sql(createTableStmt, tableName, tablePropsAsString(TABLE_PROPERTIES));
+    sql(createTableStmt, tableName(OTHER_TABLE_NAME), tablePropsAsString(TABLE_PROPERTIES));
+
+    sql(
+        "INSERT INTO %s VALUES "
+            + "('user1', 'post1', '2026-05-01', 'catA', 'p1'), "
+            + "('user2', 'post2', '2026-05-02', 'catB', 'p2'), "
+            + "('user3', 'post3', '2026-05-01', 'catA', 'p3')",
+        tableName);
+    sql(
+        "INSERT INTO %s VALUES "
+            + "('user1', 'post1', '2026-05-01', 'catA', 'p1'), "
+            + "('user2', 'post2', '2026-05-02', 'catB', 'p2'), "
+            + "('user4', 'post4', '2026-05-03', 'catC', 'p4')",
+        tableName(OTHER_TABLE_NAME));
+
+    // SELECT join on bucket source columns only (subset of partition keys)
+    assertPartitioningAwarePlan(
+        1,
+        3,
+        "SELECT t1.user_id, t1.category_id, t1.collected_date, t1.payload "
+            + "FROM %s t1 "
+            + "INNER JOIN %s t2 "
+            + "ON t1.user_id = t2.user_id AND t1.category_id = t2.category_id "
+            + "ORDER BY t1.user_id, t1.category_id",
+        tableName,
+        tableName(OTHER_TABLE_NAME));
+  }
+
+  @TestTemplate
+  public void testMergeIntoWithBucketStringSubsetOfPartitionKeys() throws NoSuchTableException {
+    // Regression test for #15349: MERGE INTO with join keys as a subset of partition keys
+    // with bucket transforms on String columns
+    String createTableStmt =
+        "CREATE TABLE %s (user_id STRING, post_id STRING, "
+            + "collected_date STRING, category_id STRING, payload STRING)"
+            + "USING iceberg "
+            + "PARTITIONED BY (collected_date, bucket(32, user_id), bucket(32, category_id))"
+            + "TBLPROPERTIES (%s)";
+
+    sql(createTableStmt, tableName, tablePropsAsString(TABLE_PROPERTIES));
+    sql(createTableStmt, tableName(OTHER_TABLE_NAME), tablePropsAsString(TABLE_PROPERTIES));
+
+    sql(
+        "INSERT INTO %s VALUES "
+            + "('user1', 'post1', '2026-05-01', 'catA', 'p1'), "
+            + "('user2', 'post2', '2026-05-02', 'catB', 'p2'), "
+            + "('user3', 'post3', '2026-05-01', 'catA', 'p3')",
+        tableName);
+    sql(
+        "INSERT INTO %s VALUES "
+            + "('user1', 'post1', '2026-05-01', 'catA', 'updated1'), "
+            + "('user2', 'post2', '2026-05-02', 'catB', 'updated2'), "
+            + "('user4', 'post4', '2026-05-03', 'catC', 'new4')",
+        tableName(OTHER_TABLE_NAME));
+
+    withSQLConf(
+        ENABLED_SPJ_SQL_CONF,
+        () ->
+            sql(
+                "MERGE INTO %s t USING %s s "
+                    + "ON t.user_id = s.user_id AND t.category_id = s.category_id "
+                    + "WHEN MATCHED THEN UPDATE SET * "
+                    + "WHEN NOT MATCHED THEN INSERT *",
+                tableName, tableName(OTHER_TABLE_NAME)));
+
+    List<Object[]> result = sql("SELECT * FROM %s ORDER BY user_id", tableName);
+    assertThat(result).hasSize(4);
+  }
+
   @TestTemplate
   public void testJoinsWithBucketingOnBinaryColumn() throws NoSuchTableException {
     checkJoin("binary_col", "BINARY", "bucket(8, binary_col)");