This is a complete list of United States senators during the 100th United States Congress listed by seniority from January 3, 1987, to January 3, 1989.
Order of service is based on the commencement of the senator's first term. Behind this is former service as a senator (only giving the senator seniority within their new incoming class), service as vice president, a House member, a cabinet secretary, or a governor of a state. The final factor is the population of the senator's state.
In this congress, Ernest Hollings (D-South Carolina) was the most senior junior senator and Daniel J. Evans (R-Washington) was the most junior senior senator.
Senators who were sworn in during the middle of the two-year congressional term (up until the last senator who was not sworn in early after winning the November 1988 election) are listed at the end of the list with no number.
The most senior senators by class were John C. Stennis (D-Mississippi) from Class 1, Strom Thurmond (R-South Carolina) from Class 2, and Daniel Inouye (D-Hawaii) from Class 3.