The 1870 Londonderry City by-election was held on 15 February. It was a ministerial by-election, triggered by the appointment of Liberal Party member of parliament Richard Dowse as Solicitor-General for Ireland. The election was strongly contested by the Irish Conservative Party at a time when such by-elections were increasingly unchallenged. During this period the Catholic population of Londonderry tended to support the Liberals while the Protestant population supported the Conservatives. Dowse won, with a slightly reduced majority, and the result has been cited as evidence for the increasing co-operation between Episcopalian and Presbyterian voters in Ulster.