In the spring of 2003, he signed with World Wrestling Entertainment seeking to realize his two lifelong goals of competing in Madison Square Garden and performing at WrestleMania. He was brought in on the heels of the signing of Rey Mysterio on the belief that he would make as big of an impact as Mysterio did upon his WWF debut. Ultimo made his WWE debut in dark match victories against Rico, Crash Holly and Shannon Moore as they debuted a series of video packages hyping the debut of Dragon on SmackDown. He made his WWE television debut at Madison Square Garden on the June 26, 2003 episode of SmackDown! in a match with Shannon Moore, where he debuted his finisher, the Asai DDT to the American wrestling audience.
Throughout the summer he competed in a tournament for the WWE United States Championship beating Jamie Noble but losing to eventual winner, Eddie Guerrero. He quickly rebounded by beating Kanyon in a WWE Heat match taped before Vengeance. He then wrestled on WWE Velocity for the next few weeks, leading to a match with Rey Mysterio on SmackDown. Dragon won the match only after Tajiri interfered and showed respect to Dragon due to their Japanese heritage. The next week Dragon teamed with Mysterio to take on Tajiri and Nunzio. After this match, he was not seen on SmackDown for several months until the build for the Cruiserweight Open at WrestleMania XX. At the event, Dragon participated in the Open. After the open, he faced Chavo Guerrero and Nunzio in his last WWE matches. His contract expired in 2004.