The Dead Kultuk (Kazakh: Өліқолтық[1] Ölıqoltyq; Russian: Мёртвый Култук) is a bay of the Caspian Sea in the coast of Kazakhstan, west of the Ustyurt desert.

[3] It had a distinct coastline in former times, but since the 1990s, with higher Caspian Sea levels, the water penetrates inland through the neck of the bay producing waterlogged marshes.

Located at the mainland end of the bay, the Kaydak Inlet cuts deep into the coast extending east and then southwards.

Nowadays both the bay and the inlet are filled with Caspian Sea water.

[4] Owing to its special colour the Dead Kultuk is the bay which appears in early maps of the Caspian Sea as 'Blue Sea' (French: Mer Bleue in maps in that language).