The 26-year-old from Victoria, who joined the club in 2009, is now committed to the Warriors until May 2015.
The South African-born van der Merwe, who has eight tries in nine appearances this season, needs just two more tries to set the club's all-time record for tries.
The Canadian speedster has 26 tries for the Warriors, 23 in the RaboDirect PRO12/Magners League and three in the Heineken Cup. Thom Evans, who holds the current club record, scored 27 tries in his Glasgow career.
"This was a pretty easy decision for me and I'm very happy to have signed the deal," van der Merwe was quoted on the club website. "The direction the squad is heading played a big factor in my decision to stay in Glasgow not just the guys who are playing here already but the guys who want to come and play their rugby here too. It's pretty exciting.
"I've been lucky enough to play in a couple of semi-finals with this club and it would be great to take it that one step further on. This squad definitely has the potential to win some silverware."
Said Glasgow head coach Gregor Townsend: "Any team would want to have a player of DTH's calibre in their ranks."
Van der Merwe is the second Canadian to extend his contract overseas in recent months.
Forward Jebb Sinclair signed a three-year extension with London Irish earlier this month.
The 26-year-old from Charters Settlement, N.Y., turned heads during a loan stint in South Africa with the Stormers and Western Province, helping WP win the 2012 Currie Cup.
The six-foot-four 252-pound Sinclair joined the Irish after the 2011 Churchill Cup.Suggest a correction