diff --git a/pkg/sql/logictest/testdata/logic_test/fk b/pkg/sql/logictest/testdata/logic_test/fk
index e8f8ab88cab8..14af5df12d3d 100644
--- a/pkg/sql/logictest/testdata/logic_test/fk
+++ b/pkg/sql/logictest/testdata/logic_test/fk
@@ -2742,3 +2742,18 @@ ALTER TABLE table1_42498 ADD FOREIGN KEY (col2, col1) REFERENCES table2_42498 (c
 
 statement ok
 DROP TABLE table1_42498, table2_42498 CASCADE
+
+# Regression test for #42680: The unique index used for the referenced columns
+# must index only those columns and no others, in order to enforce uniqueness
+# for the FK constraint.
+subtest 42680_unique_index_must_exactly_match_columns
+
+# The table has a unique index on (a, b) but not (a).
+statement ok
+CREATE TABLE target (a INT, b INT, UNIQUE INDEX (a, b));
+
+statement ok
+CREATE TABLE source (a INT, INDEX (a));
+
+statement error there is no unique constraint matching given keys for referenced table target
+ALTER TABLE source ADD FOREIGN KEY (a) REFERENCES target (a);
diff --git a/pkg/sql/sqlbase/structured.go b/pkg/sql/sqlbase/structured.go
index 487fe0fa16f2..e66ace8ab320 100644
--- a/pkg/sql/sqlbase/structured.go
+++ b/pkg/sql/sqlbase/structured.go
@@ -86,6 +86,19 @@ func (c ColumnIDs) HasPrefix(input ColumnIDs) bool {
 	return true
 }
 
+// Equals returns true if the input list is equal to this list.
+func (c ColumnIDs) Equals(input ColumnIDs) bool {
+	if len(input) != len(c) {
+		return false
+	}
+	for i := range input {
+		if input[i] != c[i] {
+			return false
+		}
+	}
+	return true
+}
+
 // FamilyID is a custom type for ColumnFamilyDescriptor IDs.
 type FamilyID uint32
 
diff --git a/pkg/sql/sqlbase/table.go b/pkg/sql/sqlbase/table.go
index 6d416a32fa1c..9175bc708e17 100644
--- a/pkg/sql/sqlbase/table.go
+++ b/pkg/sql/sqlbase/table.go
@@ -423,12 +423,12 @@ func FindFKReferencedIndex(
 ) (*IndexDescriptor, error) {
 	// Search for a unique index on the referenced table that matches our foreign
 	// key columns.
-	if ColumnIDs(referencedTable.PrimaryIndex.ColumnIDs).HasPrefix(referencedColIDs) {
+	if ColumnIDs(referencedTable.PrimaryIndex.ColumnIDs).Equals(referencedColIDs) {
 		return &referencedTable.PrimaryIndex, nil
 	}
 	// If the PK doesn't match, find the index corresponding to the referenced column.
 	for _, idx := range referencedTable.Indexes {
-		if idx.Unique && ColumnIDs(idx.ColumnIDs).HasPrefix(referencedColIDs) {
+		if idx.Unique && ColumnIDs(idx.ColumnIDs).Equals(referencedColIDs) {
 			return &idx, nil
 		}
 	}