A by-election for the seat of Polwarth in the Victorian Legislative Assembly was held on 31 October 2015. The by-election was triggered by the resignation of Terry Mulder on 3 September 2015. Former Premier of Victoria, Denis Napthine, resigned his seat representing the adjacent district of South-West Coast on the same day as Mulder. The by-election for South-West Coast was held on the same day.