The 31st United States Congress was a meeting of the legislative branch of the United States federal government, consisting of the United States Senate and the United States House of Representatives. It met in Washington, D.C., from March 4, 1849, to March 4, 1851, during the 16 months of the Zachary Taylor presidency and the first eight months of the administration of Millard Fillmore's. The apportionment of seats in this House of Representatives was based on the 1840 United States census. The Senate had a Democratic majority, while there was a Democratic plurality in the House.

Millard Fillmore was the 13th president of the United States, serving from 1850 to 1853, the last president to have been a member of the Whig Party while in office. A former member of the U.S. House of Representatives, Fillmore was elected vice president in 1848, and succeeded to the presidency when Zachary Taylor died in July 1850. Fillmore was instrumental in passing the Compromise of 1850, which led to a brief truce in the battle over the expansion of slavery.