Seagulls manager Gus Poyet had been chasing the 26-year-old for some time and he will be available for selection against Birmingham on Saturday if his international clearance is received in time.
Ulloa began his career in his homeland of Argentina, playing for San Lorenzo, Arsenal and Olimpo before making the move to Spain with Castellon.
He then scored 48 goals in 102 appearances for Almeria after joining from Castellon in 2010 and was top scorer in La Liga's second tier last season with 29 goals.
"Leo is that striker we have been looking to add for some time. He will complement our existing strikers, enjoy playing with our wingers and creative players," Poyet tolod Brighton's official website.
"He's a typical number nine box player; he's good in the air, he will hold the ball up and he has good movement around the box when the ball is wide.
"He will suit the way we play, fit in well to our style and identity and I am sure he will make us a stronger team.
"I know him from Argentina and I follow Spanish football closely so I have been aware of him for a while, but the problem was that he was not available because he didn't have a European passport.
"However, that situation has changed since the last transfer window, and as soon as we knew there was a chance of getting the passport sorted, the recruitment system moved very quickly."