The appointment, first announced in January this year, sees Rochussen succeed Tim Woolley who has moved to a non-executive position after nearly seven years leading the firm.
Rochussen was previously group chief executive of JO Hambro Capital Management, the active equity investment boutique acquired by Australian-listed fund management group BT Investment Management.
While at JOHCM he led the business as it grew from £1.5bn of equity assets under management to £23.9bn at 30 September 2016.
Before this he was chief executive of Fleming Family & Partners from 2003 to 2008.
Tom Bartlam, chairman of Polar Capital, said: “Gavin has a proven track record of successfully managing a boutique equity investment house and we look forward to him taking over the reins. He will be focused on growing the business and delivering performance to clients.”
Rochussen added: “I have known and respected Polar Capital for many years. It has a boutique culture, ethos and modus operandi with a successful team in place and the opportunity to grow, all the key ingredients that I was looking for when I wanted another challenge.”