The 35-year-old spent four-and-a-half years at Villa Park earlier on in his career, having joined from Watford, before leaving for Manchester United.
Young had spent the last 18 months in Italy with Inter Milan, where he won the Serie A title under Antonio Conte last season.
"It feels amazing to be back, it feels like I've not left," Young told VillaTV.
"You can see how much the club has evolved since my time and I'm just ready to get down to work now.
"When I heard of the interest from Aston Villa, it was straight to my agent 'get a deal done, whatever you can, get a deal done.'
"Watching from afar, seeing the squad, how well they did last season, how well they've been doing.
"The staff, the manager, how the players want to play for him; they've got that hunger, he's got that desire, that winning mentality.
"And, for me, I know I've got that winning mentality."
Villa boss Dean Smith told the club's website: "Ashley brings with him a wealth of experience of top-level football both at club and international level and he is joining having just won a league championship in Italy.
"He is a player who can play in a number of positions and, having spoken to him at length, I know he is determined to make a real impact with us this season."
Young is Villa’s second summer signing following the capture of Emiliano Buendia from Norwich City.
