The 22-year-old has risen through the ranks at Villa Park and made his first-team debut for the club against Chelsea in December 2011.
However, a series of knee injuries have curtailed his progress and he was sent out on loan to Championship side Sheffield Wednesday last season.
Gardner - who penned a new two-year deal with Villa in June - will now link up with Sami Hyypia's side in the second tier and could make his debut against Swindon Town in the League Cup later on Tuesday.
"Gary is a quality midfielder who we have been aware of for some time. When we knew he was available, we wanted to bring him here as he will give us another quality option in the middle of the park," Hyypia told Brighton's official website.
"Gary has had a tough couple of seasons due to injury, but he showed during a spell with Sheffield Wednesday last season that he is over those problems. We hope this loan spell will benefit everyone: Gary, us and Aston Villa.
"He is the type of midfielder who is active and mobile, but very importantly he has real quality on the ball - and is also capable of producing things from set pieces."
