The 33-year-old, who has made 21 appearances for England, has found himself out of favour with the Potters this season and has started just two games, one in the Capital One Cup and one in the Premier League.
He joined the Potters on a free transfer from West Ham in August 2011 after the Hammers had paid Birmingham £6m for him in January 2007.
Upson started his career at Luton before moving on to Arsenal for £2m in May 1997, but he failed to make a breakthrough with the Gunners and was loaned out to Nottingham Forest, Crystal Palace and Reading before joining the Blues.
Brighton boss Gus Poyet told the club's official Twitter feed: "We wanted a player in that position. We had the chance to bring a player of his quality and went for it.
"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: "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."