A 1831 by-election was held in the Parliament of the United Kingdom constituency of Londonderry City between 28 March and 2 April 1831. It was held after the 1830 general election in the constituency was declared void by the House of Commons following an election petition. The grounds for the petition were that the election's winner Sir Robert Ferguson, 2nd Baronet, was also the constituency's returning officer by virtue of his position as mayor of the city. Ferguson and his 1830 opponent Captain John Richard James Hart both stood in the 1831 by-election. The result was similar to that of the preceding year and Ferguson was returned. Less than a month after the by-election, Parliament was dissolved and the 1831 United Kingdom general election was called, in which Ferguson also won.