The 26-year-old Argentine has penned a two-and-a-half-year contract with the Championship club.
"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," manager Gus Poyet told Brighton's official website.
"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."
Eighth-placed Albion return to Championship action on Saturday when they travel to Birmingham City.