The 26-year-old French left-back, who spent last season on loan at Liverpool, has signed a four-year deal.
The former Lyon man earned his sole international cap in 2011 and moved to Valencia the following year, after spells with Porto and Vitoria Setubal.
He will fill a hole in defence for Villa manager Paul Lambert, who has already signed Joe Cole, Philippe Senderos and Kieran Richardson this summer.
Cissokho made 15 Premier League appearances for Premier League runners-up Liverpool last season, having won the Portuguese league title during his time at Porto, and the French Cup while at Lyon.
His first Premier League game for Liverpool was at Villa Park and he told AVTV: "Destiny has made it so that Villa Park was where I played my first game and now, one year later, I am wearing the shirt.
"I remember everything about the game, because it was my first here in England. The pitch, the stadium, the fans, everything was incredible.
"After I finished my loan with Liverpool, I was keen to come back to play football in England. I knew that Aston Villa were interested and I was very excited to come here.
"The squad we have here is very good, we have some very talented players.
"It is very exciting to meet the squad and start training with them. My mentality is always to give my best for my team mates and the fans."
Villa boss Paul Lambert said: "Aly has big game experience, he's not a novice.
"He comes across really well and he is somebody I think will be an asset. He's big, strong and powerful.
"There's great competition for places now and that's what we're trying to create.
"We've brought some new lads in and hopefully we can bring in one or two more players to make the squad even stronger."