The dean of the United States Senate is an informal term for the senator with the longest continuous service, regardless of party affiliation. This is not an official position within the Senate, although customarily (since 1945) the longest-serving member of the majority party serves as president pro tempore.
The current dean is Republican Senator Chuck Grassley of Iowa. He has been in the Senate since 1981 and became dean in 2023 with the retirement of Patrick Leahy.