The 31-year-old striker scored 26 goals in 58 games for Albion, before going on to win the Premier League title with Leicester.
He arrived at the Amex from Almeria in 2013 and commanded a club-record transfer fee received when he moved to the King Power Stadium in July 2014.
Ulloa has now re-signed until the end of the season and Albion manager Chris Hughton said, “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 he hope he will continue back here.”
The striker found the back of the net 20 times during his spell with the Foxes, notching six goals during the club’s famous 2015/16 campaign, when they won the Premier League.
Ulloa is fondly remembered for his stoppage-time winner against Nottingham Forest on the final day of the season in 2014, as his close-range header secured a play-off spot for the Seagulls.
He also was the first player to score a hat-trick at the Amex, as the Seagulls won 4-1 against Huddersfield Town.