The Councillor for Hanover and Elm Grove, Matt Follett has resigned due to moving outside of the city with his family.
A by election will be called within 35 days time excluding weekends and bank holidays.
Council leader Jason Kitcat said: "I wish Matt and his family all the very best for the future.
"I want to thank him for his hard work serving the residents of Hanover and Elm Grove ward over the past two years"
More top news
Special bus timetables will apply over the Christmas holiday in the Reading area
House prices in the south-east are expected to rise by 5% in 2015, fuelled by the recent change in Stamp Duty tax rules
Friday morning's forecast for the west of the region