The 20-year-old offers an alternative to Ahmed Elmohamady, Andre Green and Albert Adomah in the wide areas and could also play centrally if required.
Onomah has made 32 appearances for Spurs since making his debut in the FA Cup against Burnley in January 2015 and scored once.
An under-20 World Cup victor with England this summer, the youngster becomes Villa's sixth signing of the summer, following the likes of John Terry, Christopher Samba and Glenn Whelan and could play a vital role in the club's push for promotion to the Premier League.
Villa are targeting a return to the Premier League after a season in the Championship following their relegation from England's top flight during the 2015/16 season and have made a number of signings ahead of the new campaign.
Onomah is highly rated by both Tottenham staff and fans.
Onomah told avfc.co.uk: 'I can't wait to get started.
Onomah and the rest of Villa's new arrivals could make their debut this weekend, when Bruce's side welcome Hull City to Villa Park in Saturday's teatime kick-off.
The Enfield-born midfielder may have designs on breaking into Pochettino's starting line-up in the future but is now exclusively focusing on helping Aston Villa earn promotion back to England's top-flight.
The move comes following the news that Aston Villa's Jack Grealish will be out until Christmas with injury.