The 24-year-old made six Premier League starts during Paul Lambert's first season in charge at Villa Park in 2012-13, but has since fallen out of favour.
Stevens spent time on loan at both Notts County and Doncaster Rovers last term, making 13 Championship appearances for the latter.
The former Republic of Ireland Under-21 international started his career at University College Dublin before establishing himself as first-choice left-back at St Patrick's Athletic.
He then joined Shamrock Rovers, where he won two League of Ireland titles and the 2011 PFAI Young Player of the Year award before switching to Villa in January 2012.
The full-back will be available for Northampton's home clash against Burton Albion on Saturday.
‘As I was delivering the first line in my pants, all I could see in my peripheral vision was four shoulders going up and down, of Alan Shearer and Ian Wright giggling’ Gary Lineker tells FFT about the night he hosted Match of the Day in his underwear
‘It was an incredible experience to go to Liverpool. I sang a Bavarian song for my initiation but it didn’t last long – after 30 seconds I got beer over my head’: German star reveals unruly welcome at Anfield from his team-mates
