Ulloa has spent the last 18 months down the pecking order at Leicester City and at the start of last year, the Argentine expressed a desire to leave the King Power Stadium.
The 31-year-old had remained with the Foxes but after not featuring in the plans of Claude Puel, the forward has been allowed to return to his former club - where he played his football between January 2013 and July 2014 - on a loan deal until the end of the season.
Manager Chris Hughton told Brighton's official website: "We're delighted to welcome Leo back to the club. He's scored goals at Premier League level and will further strengthen our attacking options. I wasn't here during his first spell at the club, but I know he was very well thought of by all concerned, including the club's supporters.
"He's achieved a great deal since he left, including winning a Premier League title, and his experience at this level will be hugely beneficial to us.
"Albion fans will know all about him - he's a physical striker that holds the ball up well, and he's capable of scoring goals, which we hope he will continue to do back here."
Ulloa scored 26 goals in 58 appearances in all competitions during his first spell at the club.