The 31-year-old striker re-joined the Seagulls on loan from Leicester earlier today and Albion boss Chris Hughton confirmed the frontman will be in the squad on Wednesday.
Albion make the trip to St Mary's fifth from bottom in the top flight, just one point above third-from-bottom Southampton.
Fans' favourite Ulloa, who will wear shirt number 16, netted 26 goals in 58 appearances in his first spell with Brighton, before going on to win the Premier League title with Leicester.
Ulloa has made just six appearance for Leicester this season and could appear for Albion for the first time since May, 2014, at Southampton. Hughton said: "He's in the squad for Wednesday. That's all I can say and we're happy with his levels. He hasn't played as many games as what he would have liked but he's a player of very good experience and we're quite sure he'll use that."
On Ulloa's record at Brighton in his first spell at the club, Hughton said: "We know the qualities of him. Those goals were not in the Premier League at this club but he's gained great experience in the last few seasons in the games he's played, the players he's played with and games he's been involved with.
"What we wanted to do is give ourselves the best possible chance we could, particularly in the offensive areas as it's an area we've found it difficult to get the goals we need to win games.
"To be able to have that competition and them options in that position is something we definitely wanted to do.
"It's a difficult window, you can never guarantee being able to do that but we know the type of player Leo is and what he brings to the squad."