The French left-back, who spent last season on an unsuccessful loan with Liverpool, joins Paul Lambert's side on a four-year contract subject to international clearance
The club announced the news on their official Twitter page on Friday, with Cissokho securing a return to the Premier League after spending last season on loan at Liverpool.
The left-back made 19 appearances in all competitions at Anfield, including 12 league starts, as Brendan Rodgers's men finished second but largely failed to impress, with Jon Flanagan winning his place in the second half of the campaign.
That loan came after one season at Valencia, where he made 25 appearances in La Liga during 2012-13.
"I'm very happy to be here," Cissokho told Villa's official website. "I'm very proud to wear this shirt and I'm excited to play, meet my team-mates and start training."
The former Porto and Lyon man has one France cap to his name, earned in a friendly against Norway in 2010, and arrives at Villa Park to help shore up a defence that shipped 61 league goals last season.
Villa also announced on Friday that 21-year-old midfielder Daniel Johnson has signed for League One newcomers Chesterfield on a three-month loan deal.