The USS Valdemar (NCC-26198) was an Ambassador-class starship in service during the late-24th century.
In 2370, upon the Cardassian arrest of Deep Space 9 Chief of Operations Miles O'Brien, the Valdemar patrolled the border of the Cardassian Demilitarized Zone. (DS9: "Tribunal")
By 2378, the Valdemar was a part of the Starfleet Third Fleet, under the command of Admiral Christopher Truman. (Star Trek: Pioneer)
The Valdemar was a part of Task Group 9-1 at some point during the 24th century. (Starfleet Command (RPG))
The ship may have been named after Lars-Erik Valdemar, civilian commissioner of Starfleet and later United Federation of Planets Councillor in the 23rd century. (Ships of the Star Fleet)