The 21-year-old academy product, who has scored five tries in 24 senior appearances, is now tied down until the end of the 2014-15 season.
"I've benefited from my time in the academy, in particular the hours spent working on basic skills and tactical awareness," said Chisholm.
"We're a relatively young squad with great confidence and belief in each other. It is going to be another exciting year for the club and I am delighted to have re-signed."
Chisholm made his Harlequins debut during the 2010-11 campaign.