Gardner completed his medical on Monday and is in line to make his debut for Brighton in their Capital One Cup match at Swindon on Tuesday night.
The 22-year-old, whose older brother Craig plays for West Brom, came through Villa’s youth academy but saw his career stall when he suffered a second cruciate ligament injury in 2012.
Gardner returned after eight months on the sidelines and last season spent two months on loan with Sheffield Wednesday.
Brighton boss Sami Hyypia told the club's official website: "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.
"There are areas of the squad where we would like more options, and midfield was one of those areas. We have Dale Stephens out injured, but more importantly, given the considerable demands of the Championship, I want to be able to rotate the squad.
"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."