The 33-year old defender has been used rarely by Tony Pulis this season and will spend the remainder of the campaign with Gus Poyet's Seagulls.
"We wanted a player in that position. We had the chance to bring a player of his quality and went for it," Poyet told the club's Twitter page.
"He has presence, he's a leader and it's a good opportunity for us and for him to play football.
"These opportunities don't come along very often so we took it and we hope to have him ready for Saturday."
Upson added: "It's a very good opportunity for me at a time that I have not been playing. It's a chance for me to play.
"I've heard lots of positive things about Gus and his style and the whole package here at Brighton. It is very exciting."