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 that the compilers of that work make no attempt to produce a definitive list of each member's allegiances.