The 1863 United States Senate election in Wisconsin was held in the 16th Wisconsin Legislature on Incumbent Republican U.S. senator James R. Doolittle was re-elected on the first ballot.
At the start of the 1863 term, Republicans held majorities in both chambers of the Wisconsin Legislature, but their majorities were bolstered by three "pro-Union" fusion candidates, some of whom had previously been Democrats. Counting only those who were elected as Republicans, they could only afford four defections in the vote for United States senator. The incumbent, Doolittle, faced opposition from the Radical Republican wing of the party, but other candidates were reluctant to seek the nomination.
Prior to the legislative caucus, Radical Republicans pushed for the nomination of U.S. representative John F. Potter, though Potter himself did not endorse the effort. The caucus first met on the evening of January 20 and took an informal poll, finding Doolittle had the support of 43 of 72 voting members—enough for the nomination; Potter received 19 votes. The caucus adjourned for the evening and met again on January 1, at which time they took a formal vote. On the formal ballot, Doolittle received 53 votes and was declared the party's nominee.
The 16th Wisconsin Legislature met in joint session on January 22, 1863, to elect a U.S. senator. The voting went along party lines, with only one Republican member absent. James R. Doolittle received the votes of all other Republicans and Union legislators, winning the election.