The Wairau by-election 1875 was a by-election held in the Wairau electorate in the Marlborough Province during the 5th New Zealand Parliament, on 21 June 1875. The by-election was caused by the resignation of incumbent MP Arthur Seymour and was won by Joseph Ward, who defeated William Sefton Moorhouse. Ward was a well-known politician in Marlborough. Moorhouse had political seniority over Ward and was at the time Mayor of Wellington, but had no personal connection to Marlborough.