No it wasn't...the Leviathan was once a republic class vessel.
As with the other capital ships of the Sith fleet, the Leviathan was originally a Republic ship that fell into Sith hands after the Battle of Malachor V. Apparently a distant technological ancestor of the later Star Destroyers, the Leviathan was one of many similarly designed ships commissioned by the Republic for the Mandalorian Wars. The fact that the only ship to survive Malachor V and not go with Revan, the Ravager, bears a similar but not identical design to the Leviathan, indicates that the Sith's ex-Republic ships may have been modified by the Star Forge. - Star Wars Wikipedia