diff --git a/CHANGELOG b/CHANGELOG
index 60d907a8c..89bbddcc5 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,7 @@
 === master
 
+* Support MERGE WHEN NOT MATCHED BY SOURCE on PostgreSQL 17+ (jeremyevans)
+
 * Add stdio_logger extension for a minimal logger that Sequel::Database can use (jeremyevans)
 
 * Simplify Database#inspect output to database_type, host, database, and user (jeremyevans)
diff --git a/lib/sequel/adapters/shared/postgres.rb b/lib/sequel/adapters/shared/postgres.rb
index 661932662..4087ebba2 100644
--- a/lib/sequel/adapters/shared/postgres.rb
+++ b/lib/sequel/adapters/shared/postgres.rb
@@ -2068,6 +2068,19 @@ def merge(&block)
         end
       end
 
+      # Return a dataset with a WHEN NOT MATCHED BY SOURCE THEN DELETE clause added to the
+      # MERGE statement.  If a block is passed, treat it as a virtual row and
+      # use it as additional conditions for the match.
+      #
+      #   merge_delete_not_matched_by_source
+      #   # WHEN NOT MATCHED BY SOURCE THEN DELETE
+      #
+      #   merge_delete_not_matched_by_source{a > 30}
+      #   # WHEN NOT MATCHED BY SOURCE AND (a > 30) THEN DELETE
+      def merge_delete_when_not_matched_by_source(&block)
+        _merge_when(:type=>:delete_not_matched_by_source, &block)
+      end
+
       # Return a dataset with a WHEN MATCHED THEN DO NOTHING clause added to the
       # MERGE statement.  If a block is passed, treat it as a virtual row and
       # use it as additional conditions for the match.
@@ -2094,6 +2107,19 @@ def merge_do_nothing_when_not_matched(&block)
         _merge_when(:type=>:not_matched, &block)
       end
 
+      # Return a dataset with a WHEN NOT MATCHED BY SOURCE THEN DO NOTHING clause added to the
+      # MERGE BY SOURCE statement.  If a block is passed, treat it as a virtual row and
+      # use it as additional conditions for the match.
+      #
+      #   merge_do_nothing_when_not_matched_by_source
+      #   # WHEN NOT MATCHED BY SOURCE THEN DO NOTHING
+      #
+      #   merge_do_nothing_when_not_matched_by_source{a > 30}
+      #   # WHEN NOT MATCHED BY SOURCE AND (a > 30) THEN DO NOTHING
+      def merge_do_nothing_when_not_matched_by_source(&block)
+        _merge_when(:type=>:not_matched_by_source, &block)
+      end
+
       # Support OVERRIDING USER|SYSTEM VALUE for MERGE INSERT.
       def merge_insert(*values, &block)
         h = {:type=>:insert, :values=>values}
@@ -2103,6 +2129,19 @@ def merge_insert(*values, &block)
         _merge_when(h, &block)
       end
     
+      # Return a dataset with a WHEN NOT MATCHED BY SOURCE THEN UPDATE clause added to the
+      # MERGE statement.  If a block is passed, treat it as a virtual row and
+      # use it as additional conditions for the match.
+      #
+      #   merge_update_not_matched_by_source(i1: Sequel[:i1]+:i2+10, a: Sequel[:a]+:b+20)
+      #   # WHEN NOT MATCHED BY SOURCE THEN UPDATE SET i1 = (i1 + i2 + 10), a = (a + b + 20)
+      #
+      #   merge_update_not_matched_by_source(i1: :i2){a > 30}
+      #   # WHEN NOT MATCHED BY SOURCE AND (a > 30) THEN UPDATE SET i1 = i2
+      def merge_update_when_not_matched_by_source(values, &block)
+        _merge_when(:type=>:update_not_matched_by_source, :values=>values, &block)
+      end
+
       # Use OVERRIDING USER VALUE for INSERT statements, so that identity columns
       # always use the user supplied value, and an error is not raised for identity
       # columns that are GENERATED ALWAYS.
@@ -2305,10 +2344,9 @@ def _merge_insert_sql(sql, data)
         _insert_values_sql(sql, values)
       end
 
-      def _merge_matched_sql(sql, data)
+      def _merge_do_nothing_sql(sql, data)
         sql << " THEN DO NOTHING"
       end
-      alias _merge_not_matched_sql _merge_matched_sql
 
       # Support MERGE RETURNING on PostgreSQL 17+.
       def _merge_when_sql(sql)
diff --git a/lib/sequel/dataset/sql.rb b/lib/sequel/dataset/sql.rb
index 6b9a95d7c..189e0eb88 100644
--- a/lib/sequel/dataset/sql.rb
+++ b/lib/sequel/dataset/sql.rb
@@ -901,18 +901,31 @@ def _merge_delete_sql(sql, data)
     MERGE_TYPE_SQL = {
       :insert => ' WHEN NOT MATCHED',
       :delete => ' WHEN MATCHED',
+      :delete_not_matched_by_source => ' WHEN NOT MATCHED BY SOURCE',
       :update => ' WHEN MATCHED',
+      :update_not_matched_by_source => ' WHEN NOT MATCHED BY SOURCE',
       :matched => ' WHEN MATCHED',
       :not_matched => ' WHEN NOT MATCHED',
+      :not_matched_by_source => ' WHEN NOT MATCHED BY SOURCE',
     }.freeze
     private_constant :MERGE_TYPE_SQL
 
+    MERGE_NORMALIZE_TYPE_MAP = {
+      :delete_not_matched_by_source => :delete,
+      :update_not_matched_by_source => :update,
+      :matched => :do_nothing,
+      :not_matched => :do_nothing,
+      :not_matched_by_source => :do_nothing,
+    }.freeze
+    private_constant :MERGE_NORMALIZE_TYPE_MAP
+
     # Add the WHEN clauses to the MERGE SQL
     def _merge_when_sql(sql)
       raise Error, "no WHEN [NOT] MATCHED clauses provided for MERGE" unless merge_when = @opts[:merge_when]
       merge_when.each do |data|
         type = data[:type]
         sql << MERGE_TYPE_SQL[type]
+        type = MERGE_NORMALIZE_TYPE_MAP[type] || type
         _merge_when_conditions_sql(sql, data)
         send(:"_merge_#{type}_sql", sql, data)
       end
diff --git a/spec/adapters/postgres_spec.rb b/spec/adapters/postgres_spec.rb
index 9307f30f0..1239f1d32 100644
--- a/spec/adapters/postgres_spec.rb
+++ b/spec/adapters/postgres_spec.rb
@@ -6030,3 +6030,48 @@ def check(ds)
     @m1.all.must_equal []
   end
 end if DB.server_version >= 170000
+
+describe "MERGE WHEN NOT MATCHED BY SOURCE" do
+  before(:all) do
+    @db = DB
+    @db.create_table!(:m1){Integer :i1; Integer :a}
+    @db.create_table!(:m2){Integer :i2}
+    @m1 = @db[:m1]
+  end
+  after do
+    @m1.delete
+  end
+  after(:all) do
+    @db.drop_table?(:m1, :m2)
+  end
+
+  it "should allow inserts, updates, do nothings, and deletes based on conditions in a single MERGE statement" do
+    ds = @m1.merge_using(:m2, :i1=>:i2)
+
+    @m1.insert(1, 2)
+    @m1.all.must_equal [{:i1=>1, :a=>2}]
+
+    ds.merge_do_nothing_when_not_matched_by_source.merge
+    @m1.all.must_equal [{:i1=>1, :a=>2}]
+
+    ds.merge_update_when_not_matched_by_source(:i1=>Sequel[:i1]+10, :a=>Sequel[:a]+20).merge
+    @m1.all.must_equal [{:i1=>11, :a=>22}]
+
+    ds.merge_delete_when_not_matched_by_source.merge
+    @m1.all.must_equal []
+
+    @m1.insert(1, 100)
+    @m1.insert(2, 40)
+    @m1.insert(3, 20)
+    @m1.insert(4, 5)
+
+    # conditions
+    ds.
+      merge_do_nothing_when_not_matched_by_source{a > 50}.
+      merge_delete_when_not_matched_by_source{a > 30}.
+      merge_update_when_not_matched_by_source(:a=>Sequel[:a]+20){a > 10}.
+      merge_update_when_not_matched_by_source(:a=>Sequel[:a]-20).
+      merge
+    @m1.order(:i1).all.must_equal [{:i1=>1, :a=>100}, {:i1=>3, :a=>40}, {:i1=>4, :a=>-15}]
+  end
+end if DB.server_version >= 170000