The 27-year-old Belgian joins Villa for an undisclosed fee, having last season helped Leicester win the Premier League title while also being part of the Middlesbrough squad that won promotion to the top flight.
De Laet spent the first half of the campaign at the King Power Stadium, making 12 league appearances for the title-winning Foxes, before joining Championship side Boro on loan in February.
In total De Laet made 129 appearances for Leicester in four years after joining from Manchester United, while also winning promotion to the Premier League with the Foxes in 2014
Villa m anager Roberto Di Matteo said: "I'm very pleased to have brought Ritchie to the club
"He is another player who not only has Championship experience but has also played a part in helping a team win promotion back to the Premier League."
Source : PA
Source: PA