The 22-year-old tighthead signed his first professional contract at Franklin’s Gardens last season after impressing while on loan from Championship outfit Ampthill.
Garside became Saint number 1997 in October 2018 when he made his full debut against Bristol Bears in the Premiership Rugby Cup.
Since then, he has notched up six first-team appearances for Saints, including two in the Gallagher Premiership.
And Garside, who began his rugby career at Oaklands College and featured in the RFU’s AASE League, is delighted to be sticking around at Saints.
“I’m absolutely delighted to extend my stay with Northampton Saints,” he said.
“This is a great club with a fantastic playing environment, and I feel like I am growing as a rugby player every day here.
“Getting the opportunity last season to play rugby at the very top level was a huge breakthrough for me.
"I’m really excited about the prospect of signing on again so I can push for a regular place in the side.”