Ultimate Swampfire is Ben's ultimate form of Swampfire. It was first used by Ben to fight Vilgax in The Final Battle: Part 2, making it Ben's first ultimate transformation, a reference to Ben's transformation to regular Swampfire in Ben 10: Alien Force, being his first transformation when puts on the Omnitrix again. He will appear in Ultimate Alien, as one of Ben's new aliens. He is voiced by Dee Bradley Baker.
After the induced evolution, Swampfire now appears as humanoid tree with three blue gooey shells containing an organic, blue-colored kerosene gel-like solution, (as said by Dwayne McDuffie) that Ultimate Swampfire shoots out and ignites, creating more intense, blue-colored flames. He is the same height as swampfire. He has many of the same abilities. His Ultimatrix sign is on his chest. He also has the same feet as Swampfire.