List of United States senators in the 51st Congress

This is a complete list of members of the United States Senate during the 51st United States Congress listed by seniority, from March 4, 1889, to March 3, 1891.

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 his or her 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.

Senators who were sworn in during the middle of the Congress (up until the last senator who was not sworn in early after winning the November 1890 election) are listed at the end of the list with no number.