SS Castilian was a British cargo steamship and is now a dangerous wreck in the Irish Sea off the coast of North Wales.

In 1943 while carrying munitions she struck rocks off The Skerries, Isle of Anglesey and sank.

[6] In the Second World War Castilian sailed between Britain, Gibraltar and Allied ports in the Mediterranean from September 1939 until June 1940.

In 1987 a Royal Navy clearance vessel spent several months removing unexploded ordnance from nearby Fydlyn Bay that was believed to have come from the wreck.

An exclusion zone prohibits scuba diving within 500-metre (1,600 ft) of the wreck.