Balsillie held 26.8 million shares in BlackBerry, formerly Research in Motion, after he stepped down stepped down as CEO a year ago, when he was replaced by Thorsten Heins.
The news sent shares in BlackBerry down 3.4% to $13.52 (£8.71) in early trading on the Nasdaq. Shares in the Canadian firm also fell in Toronto by 3.5% to C$13.51 (£8.69).
The dumping of such a large stake by Balsillie, who was one of the largest individual shareholders in BlackBerry, is unlikely to inspire confidence in the firm that badly needs its new BlackBerry 10 operating system and Z10 phone to be a hit.
Balsillie had run BlackBerry with fellow former co-CEO Mike Lazaridis who is still at the company as vice-chairman. The pair stepped aside for Heins after shareholder criticism over the future direction of the company.
Some have praised the BlackBerry's new Z10 smartphone, which has received mixed reviews, but it faces a tough battle as it attempts to take on Apple and Samsung.Follow @gordonmacmillan