The midfielder has moved in a deal worth around Â£3million and has penned a four-and-a-half year deal
Lansbury was Forest's captain but had not played since December having been out injured before ex-boss Philippe Montanier felt he was not in the right frame of mind to feature
He told the club's official site: "I am delighted to be here at Aston Villa.
"As soon as I knew Aston Villa were interested, I wanted to come here.
"It's the size and reputation of the football club that attracted me
It's a big, big name in the English game and I can't wait to get started here now."
The transfer dragged because of Forest's protracted takeover, which collapsed last week, and Villa were prepared to wait until the summer to get their man, with Lansbury out of contract at the end of the season
The 26-year-old joined Forest from Arsenal in 2012 and made 150 appearances having also spent time on loan at Norwich, West Ham and Watford
He trained with his new team-mates on Friday and could make his debut at home to Preston on Saturday with Villa 13th in the Sky Bet Championship
Source : PA