This is a complete list of United States senators during the 1st United States Congress listed by seniority from March 4, 1789, to March 3, 1791.
The order of service is based on the commencement of the senator's first term, with senators entering service the same day ranked alphabetically. The Senate now assigns an official number to each senator, which is the second number given in the table.
In this congress, James Gunn was the most senior junior senator and Theodore Foster was the most junior senior senator.
During this time, there were no official parties, but senators are labeled as Pro-Administration (F, Federalist), and Anti-Administration (A).