Forster-Caskey played six times for the Seagulls last season, scoring two goals for the Championship club.
In May 2010, the highly-rated 18-year-old became the youngest player to appear for Brighton when he came on against Yeovil on the last day of the season.
Forster-Caskey has represented England at Under-16, Under-17 and Under-18 level.
U's boss Chris Wilder told the club's official website: "We've worked very hard to get Jake and have got him until January which is really good. He's a very exciting young player.
"He's a left-footed midfield player that gets forward, likes to get into the box and is very highly rated at Brighton. He's just signed a new contract there and hopefully he can add to a lot of the talent that we already have here in central midfield."