diff --git a/src/main/java/net/sf/jsqlparser/statement/merge/MergeSide.java b/src/main/java/net/sf/jsqlparser/statement/merge/MergeSide.java
index d871021f7..6dc7815fb 100644
--- a/src/main/java/net/sf/jsqlparser/statement/merge/MergeSide.java
+++ b/src/main/java/net/sf/jsqlparser/statement/merge/MergeSide.java
@@ -29,6 +29,29 @@ public enum MergeSide {
      * @throws IllegalArgumentException if {@code image} is neither {@code TARGET} nor
      *         {@code SOURCE}
      */
+    /**
+     * Validates the clause pairing of a {@code WHEN NOT MATCHED} branch: {@code BY TARGET} (the
+     * default) only allows an {@code INSERT} clause, {@code BY SOURCE} only allows {@code UPDATE}
+     * or {@code DELETE}.
+     *
+     * @param side the parsed {@code BY TARGET}/{@code BY SOURCE} qualifier, null when absent
+     * @param operation the parsed clause
+     * @return the passed {@code operation}
+     * @throws IllegalArgumentException when the pairing is not legal in any dialect
+     */
+    public static MergeOperation validatePairing(MergeSide side, MergeOperation operation) {
+        if (side == MergeSide.SOURCE) {
+            if (operation instanceof MergeInsert) {
+                throw new IllegalArgumentException(
+                        "WHEN NOT MATCHED BY SOURCE cannot take an INSERT clause");
+            }
+        } else if (!(operation instanceof MergeInsert)) {
+            throw new IllegalArgumentException(
+                    "WHEN NOT MATCHED [BY TARGET] cannot take an UPDATE or DELETE clause");
+        }
+        return operation;
+    }
+
     public static MergeSide fromImage(String image) {
         for (MergeSide value : values()) {
             if (value.name().equalsIgnoreCase(image)) {
diff --git a/src/main/jjtree/net/sf/jsqlparser/parser/JSqlParserCC.jjt b/src/main/jjtree/net/sf/jsqlparser/parser/JSqlParserCC.jjt
index d86e227d7..9d0314204 100644
--- a/src/main/jjtree/net/sf/jsqlparser/parser/JSqlParserCC.jjt
+++ b/src/main/jjtree/net/sf/jsqlparser/parser/JSqlParserCC.jjt
@@ -4489,7 +4489,7 @@ MergeOperation MergeWhenNotMatched() : {
         |
         operation = MergeDeleteClause(predicate) { ((MergeDelete) operation).setSide(side); }
     )
-    { return operation; }
+    { return MergeSide.validatePairing(side, operation); }
 }
 
 // table names seem to allow ":" delimiters, e.g. for Informix see #1134
diff --git a/src/test/java/net/sf/jsqlparser/statement/merge/MergeTest.java b/src/test/java/net/sf/jsqlparser/statement/merge/MergeTest.java
index 27c8c4da0..87d1d1448 100644
--- a/src/test/java/net/sf/jsqlparser/statement/merge/MergeTest.java
+++ b/src/test/java/net/sf/jsqlparser/statement/merge/MergeTest.java
@@ -25,6 +25,7 @@
 import static net.sf.jsqlparser.test.TestUtils.assertOracleHintExists;
 import static net.sf.jsqlparser.test.TestUtils.assertSqlCanBeParsedAndDeparsed;
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.mockito.Mockito.mock;
 
 /**
@@ -363,4 +364,36 @@ private static Stream<Arguments> deriveStandardClausesFromOperationsCases() {
                 Arguments.of(Arrays.asList(insert1, insert2, update1, update2, delete1), update1,
                         insert1, true));
     }
+
+    @Test
+    void testMergeRejectsInvalidWhenNotMatchedClausePairings() {
+        // WHEN MATCHED rows exist in the target: INSERT is not legal
+        assertThrows(JSQLParserException.class, () -> CCJSqlParserUtil.parse(
+                "MERGE INTO t USING s ON t.id = s.id WHEN MATCHED THEN INSERT (a) VALUES (1)"));
+
+        // WHEN NOT MATCHED [BY TARGET] rows do not exist in the target: only INSERT is legal
+        assertInvalidMergePairing(
+                "MERGE INTO t USING s ON t.id = s.id WHEN NOT MATCHED THEN UPDATE SET a = 1");
+        assertInvalidMergePairing(
+                "MERGE INTO t USING s ON t.id = s.id WHEN NOT MATCHED THEN DELETE");
+        assertInvalidMergePairing(
+                "MERGE INTO t USING s ON t.id = s.id WHEN NOT MATCHED BY TARGET THEN UPDATE SET a = 1");
+        assertInvalidMergePairing(
+                "MERGE INTO t USING s ON t.id = s.id WHEN NOT MATCHED BY TARGET THEN DELETE");
+
+        // WHEN NOT MATCHED BY SOURCE rows exist in the target: INSERT is not legal
+        assertInvalidMergePairing(
+                "MERGE INTO t USING s ON t.id = s.id WHEN NOT MATCHED BY SOURCE THEN INSERT (a) VALUES (1)");
+    }
+
+    private static void assertInvalidMergePairing(String sql) {
+        JSQLParserException exception = assertThrows(JSQLParserException.class,
+                () -> CCJSqlParserUtil.parse(sql));
+        Throwable cause = exception;
+        while (cause.getCause() != null) {
+            cause = cause.getCause();
+        }
+        assertThat(cause).isInstanceOf(IllegalArgumentException.class);
+        assertThat(cause.getMessage()).contains("WHEN NOT MATCHED");
+    }
 }