The 80th United States Congress was a meeting of the legislative branch of the United States federal government, composed of the United States Senate and the United States House of Representatives. It met in Washington, D.C. from January 3, 1947, to January 3, 1949, during the third and fourth years of Harry S. Truman's presidency. The apportionment of seats in this House of Representatives was based on the 1940 United States census.

The Republicans won the majority in both chambers, marking the first time since the 71st Congress they held full control of Congress, and the first time since the 72nd Congress they held either of the two chambers. This also ended a 14-year Democratic overall federal government trifecta, dating back to the 73rd Congress.

Although the 80th Congress passed a total of 906 public bills, President Truman nicknamed it the "Do Nothing Congress" and, during the 1948 election, campaigned as much against it as against his formal opponent, Thomas E. Dewey. The 80th Congress passed several significant bills with bipartisan support, most famously the Truman Doctrine, the Marshall Plan, and the Taft–Hartley Act, but it opposed most of Truman's Fair Deal bills.