The Samurai Blue midfielder becomes one of the highest-profile Japanese players to move to South Korea's top flight, after agreeing a loan spell with the 2011 runners-up
The 25-year-old Japanese midfielder, who joined Mallorca in December 2010 on a five-year deal, had struggled for opportunities with the Islander club this term.
As a result, Mallorca have loaned him to Ulsan, who finished the 2011 season as K-League runners-up.
Former Gamba Osaka man Ienaga told reporters: "I am very happy to play for a club with such facilities. I will try my best for the success of the club and myself."
Ulsan boss Kim Ho-Gon added: "I was looking for a attacking midfielder with passing abilities. He will help us with our attack."
It had been reported by Sports Seoul earlier in the week, that the proposed deal could be derailed as Mallorca wanted Ulsan to pay for player insurance which is unprecedented in Korea. However the deal has gone ahead.