Sometime on or before February 2, 2020, a by-election must be held for the seat in the Legislative Assembly of Ontario for the electoral district of Ottawa-Vanier.

On August 2, the Chief Electoral Officer of Ontario received a notice of vacancy for the position from the Assembly.

MPP Nathalie Des Rosiers, who had announced her intention to leave office on May 16, resigned on July 31 to begin working August 1 as the Principal of Massey College at the University of Toronto.

Any by-election in the province must be called within six months of the Chief Electoral Officer receiving notice of the vacancy.