Ulloa has been on the fringes at Leicester, playing only 18 minutes across four substitute appearances in the Premier League this term.
The 31-year-old scored 26 goals in 58 appearances in his previous spell for Brighton, who have already broken their club record to sign PSV striker Jurgen Locadia this month.
Seagulls boss Chris Hughton is thrilled to be bringing Ulloa back to the club, three-and-a-half years after the Argentine left for Leicester.
"We're delighted to welcome Leo back to the club," said Hughton, on the club's official website.
"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 hit 20 goals in 102 games for Leicester, but last scored for the club in December 2016 in a 2-2 draw at Stoke City.