Middlesex West was a federal electoral district represented in the Canadian House of Commons from 1867 to 1968. It was located in the province of Ontario. It was created by the British North America Act of 1867 which divided the County of Middlesex into three ridings: Middlesex North, Middlesex West and Middlesex East.
The West Riding initially consisted of the Townships of Delaware, Caradoc, Metcalfe, Mosa and Ekfrid, and the Village of Strathroy.
In 1882, it was redefined to include the townships of Adelaide, Euphemia, the villages of Glencoe, Newbury and Wardsville, and to exclude the township of Carradoc.
In 1903, it was redefined to consist of the townships of Caradoc, Delaware, Ekfrid, Metcalfe and Mosa, the town of Strathroy, and the villages of Glencoe, Newbury and Wardsville.
In 1914, it was redefined to consist of the townships of Adelaide, Lobo, Delaware, Caradoc, Metcalfe, Mosa, Ekfrid, McGillivray, Williams East and Williams West, the towns of Park Hill and Strathroy, and the villages of Ailsa Craig, Glencoe, Newbury and Wardsville.