List of United Kingdom by-elections (1801–1806)

This is a list of parliamentary by-elections in the United Kingdom held between 1801 and 1806, with the names of the previous incumbent and the victor in the by-election.

In the absence of a comprehensive and reliable source for party and factional alignments in this period, no attempt is made to define them in this article. The House of Commons: 1790–1820 provides some guidance to the complex and shifting political relationships, but it is significant^{[according to whom?]} that the compilers of that work make no attempt to produce a definitive list of each member's allegiances.

Where the cause of by-election is given as "resigned", this indicates that the incumbent was appointed on his own request to an "office of profit under the Crown". Offices used, in this period, were the Stewards of the Chiltern Hundreds or the Manor of East Hendred and the Escheators of Munster or Ulster. These appointments are made as a constitutional device for leaving the House of Commons, whose Members are not permitted to resign.

The c/u column denotes whether the by-election was a contested poll or an unopposed return. If the winner was re-elected, at the next general election and any intermediate by-elections, this is indicated by an * following the c or u. In a few cases the winner was elected at the next general election but had not been re-elected in a by-election after the one noted. In those cases no * symbol is used.

An incumbent who vacated a seat in the Irish Parliament, before the United Kingdom of Great Britain and Ireland came into existence on 1 January 1801, has (IP) following their name. In two cases two member seats in the Parliament of Ireland were reduced to single member constituencies in the United Kingdom Parliament, with both old seats being vacant at the start of the union and a by-election having to be held to fill the vacancy for the new seat.

Where a seat was vacated, because the incumbent member inherited a peerage or was created a peer or was appointed to an office, click the highlighted cause for further details of the particular circumstances. If the by-election date is highlighted, this is a link to an article about the by-election.

• Members of the 1st UK Parliament from Ireland
• Members of the 2nd UK Parliament from Ireland

• List of MPs since 1660^[usurped]
• Return of the name of every member of the lower house of parliament of England, Scotland, and Ireland, with name of constituency represented, and date of return, from 1213 to 1874.
• Walker, Brian M., ed. (1978). Parliamentary Election Results in Ireland, 1801–1922. Royal Irish Academy. ISBN 0-901714-12-7
• The House of Commons 1790–1820, edited by R.G. Thorne (Secker & Warburg 1986) ISBN 0-436-52101-6

• History of Parliament: Members 1790–1820
• History of Parliament: Constituencies 1790–1820