HDMS Sælen (S323) (or KDM Sælen ) is one of the three Tumleren-class small coastal submarines of the Royal Danish Navy.

She was built as a Type 207 submarine by Rheinstahl-Nordseewerke of Emden, Germany in 1965 for the Royal Norwegian Navy and served for 25 years as HNoMS Uthaug before being purchased by Denmark in 1990, and renamed after the seal. Her international call sign is OUCJ.

On 4 December 1990, Sælen sank while being towed from Copenhagen to Aarhus. On 17 December, she was raised by the German floating crane Roland and taken to Aarhus for repair. On 10 August 1993, diving approval was issued and the boat was recommissioned.

Sælen served in the 2003 invasion of Iraq from May 2002 until June 2003. To speed her return to her homeport of Frederikshavn after 385 days of deployment in the Mediterranean Sea and the Persian Gulf, she was transported on board the heavy-lift ship Grietje.