The Hanover and Elm Grove by-election will take place on Thursday 11 July.
The Conservative Party has picked a 54-year-old former civil servant to run for the seat on Brighton and Hove City Council.
Robert Knight, from Hollingdean, used to work at the Department for Work and Pensions.
The seat became vacant when Green councillor Matt Follett, a lecturer at Brighton University, stood down last week.
The Green Party has picked David Gibson, a Hanover resident and community activist, to try to keep the seat.
At the last local elections in May 2011 the Greens won all three seats in Hanover and Elm Grove which was a solid Labour ward at one time.
The Labour Party is due to select its candidate on Saturday (1 June).