The 31-year-old striker joined Leicester from Brighton in July 2014 and has agreed a deal to go back to the Amex Stadium until the end of the season.
The Argentinian is Brighton’s third signing of the January transfer window after Jurgen Locadia (PSV Eindhoven) and Warren O’Hora (Bohemians).
"We’re delighted to welcome Leo back to the club," Brighton manager Chris Hughton told Brighton's official website. "He’s scored goals at Premier League level and will further strengthen our attacking options."
Ulloa will be looking to make his second Brighton debut in their next fixture, at Southampton, on Wednesday.