The agency has been asked to reposition the 138-year-old brand.
The pitch process kicked off in February. Grey replaces the incumbent, Forsman & Bodenfors, which did not repitch for the business.
Grey London’s partner agencies Grey Shopper, Grey Possible and Grey Works have also been appointed to the account. Work will span multiple channels including activation, experiential, TV, digital and point-of-sale.
Grey’s first campaign will launch in September 2016 and focus on the company’s autumn/winter range.
Helly Hansen’s senior vice-president of brand and marketing, Erik Burbank, said: "Helly Hansen has always had a strong emotional bond with demanding professionals. The ideas that Grey presented will help us to forge that same bond with an even greater number of consumers."