The USS Valdemar was a Federation starship that was in service during the late 24th century.
In 2370 the Valdemar was dispatched to the border of the Cardassian Demilitarized Zone, following the arrest of Starfleet officer Miles O'Brien. The Enterprise-D and the Prokofiev were also ordered to patrol the border of the DMZ. (DS9: "Tribunal")
Background
According to the Star Trek Encyclopedia this vessel was an Ambassador-class starship, with a registry NCC-26198.
The Valdemar may have been named for one of the historical kings of Denmark, and some have speculated that it was named after the fictional land created by Mercedes Lackey. However, the Star Trek Encyclopedia states that it was most likely named for Valdemar Poulsen, the inventor of the first tape recorder.