Sammisoq (old spelling Sangmissoq, Danish: Christian IV Ø; English traditional:Christian IV Island) is an island in the Kujalleq municipality in southern Greenland, located northeast of Uummannarsuaq cape.

It is separated from mainland Greenland to the north by the narrow Prince Christian Sound.

Its coast is deeply indented and its southern part is almost separated from the main island by two narrow inlets.

[2] To the south lies Itilleq Island and to the west Annikitsoq, both part of the same group.

[4][failed verification] It is an ultra prominent peak, meaning a journey to any higher mountain involves descending more than 1500 m down before going up again, in this case down to sea level.