USCGC Blackthorn (WLB-391) was a 180-foot (55 m) seagoing buoy tender (WLB) which sank in 1980 in a collision near the Tampa Bay Sunshine Skyway Bridge, resulting in 23 crew member fatalities.

An Iris-class vessel, she was built by Marine Ironworks and Shipbuilding Corporation in Duluth, Minnesota. Blackthorn's preliminary design was completed by the United States Lighthouse Service and the final design was produced by Marine Iron and Shipbuilding Corporation in Duluth. On 21 May 1943 the keel was laid, she was launched on 20 July 1943 and commissioned on 27 March 1944. The original cost for the hull and machinery was $876,403.

Blackthorn was one of 39 original 180-foot (55 m) seagoing buoy tenders built between 1942 and 1944. All but one of the original tenders, USCGC Ironwood (WLB-297), were built in Duluth.

Blackthorn was initially assigned to the Great Lakes for ice-breaking duties, but after only a few months, she was reassigned to San Pedro, California. She served in San Pedro for several years before being brought into the gulf coast region to serve in Mobile, Alabama then transferred to Galveston, Texas for the final years of her service until the accident.

In 1979–1980, Blackthorn underwent a major overhaul in Tampa, Florida. However, on 28 January 1980, while leaving Tampa Bay after the completion of the overhaul, she collided with the tanker SS Capricorn. Shortly after the collision, Blackthorn capsized, killing 23 of her crew. The cutter was raised for the investigation, but ultimately was scuttled in the Gulf of Mexico after the investigation was complete. She currently serves as an artificial reef for recreational diving and fishing.