The 100th 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, 1987, to January 3, 1989, during the last two years of Ronald Reagan's presidency. The apportionment of seats in the House of Representatives was based on the 1980 United States census.

This is the most recent Congress with a Democratic senator from the state of Mississippi, John C. Stennis, who retired at the end of Congress, and a Republican senator from the state of Connecticut, Lowell Weicker, who lost re-election in 1988.

Both chambers had a Democratic majority, with the Democrats increasing their lead in the House, and regaining the Senate for the first time since the 96th Congress from 1979 to 1981.

This list is arranged by chamber, then by state. Senators are listed by class, and representatives are listed by district.

Senators are popularly elected statewide every two years, with one-third beginning new six-year terms with each Congress, In this Congress, Class 1 meant their term ended with this Congress, facing re-election in 1988; Class 2 meant their term began in the last Congress, facing re-election in 1990; and Class 3 meant their term began in this Congress, facing re-election in 1992.

Lists of committees and their party leaders for members of the House and Senate committees can be found through the Official Congressional Directory at the bottom of this article. The directory after the pages of terms of service lists committees of the Senate, House (Standing with Subcommittees, Select and Special) and Joint and, after that, House/Senate committee assignments. On the committees section of the House and Senate in the Official Congressional Directory, the committee's members on the first row on the left side shows the chairman of the committee and on the right side shows the ranking member of the committee.