A by-election for the seat of Katanning in the Western Australian Legislative Assembly was held on 31 August 1935, following the death of the sitting member, Arnold Piesse of the Country Party. Six candidates contested the election, including three endorsed Country candidates. Neither of the two other major parties, Labor and the Nationalists, fielded candidates. The campaign focused mainly on local issues, and no candidate received more than a quarter of the vote. With preferential voting in use, endorsed Country Party candidate Arthur Watts was elected over unendorsed Country Party candidate Nelson Lemmon after five rounds of counting, beginning Watts' 27-year career in the Legislative Assembly.