This is a complete list of United States senators during the 90th United States Congress listed by seniority from January 3, 1967, to January 3, 1969.
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, J. William Fulbright (Arkansas) was the senior junior senator and Harry F. Byrd Jr. (Virginia) was the 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 1968 election) are listed at the end of the list with no number.
The senior senators by class were Spessard Holland (D-Florida) from Class 1, Richard Russell Jr. (D-Georgia) from Class 2, and Carl Hayden (D-Arizona) from Class 3.