Both U.S. Senate seats will be on the ballot in Oklahoma. U.S. Sen. Jim Inhofe, R-Tulsa, is running for re-election.
Coburn's resignation will become effective early next year when his replacement is sworn in.
Coburn had limited himself to two terms in the Senate, so he would have been retiring in 2016 if he hadn't decided to leave early. His replacement will be elected to complete the last two years of his term.
A physician, Coburn won a U.S. House seat in 1994 and served until 2001, leaving because he had limited himself to three terms there.
He ran for the Senate in 2004, entering the race relatively late, but he still easily beat the Republican field and then defeated former U.S. Rep. Brad Carson in the general election.
In the House and Senate, Coburn relentlessly attacked spending of all kinds from both parties, and likely had more influence than any other member in killing off the pork barrel system that gave lawmakers millions of dollars each to spread around their districts.
He has engaged in battles big and small over spending, blocking new programs that he considered duplicative and proposing major changes to Medicare and Social Security. His annual Wastebook is a compendium of spending items
Inhofe said Thursday night, “I was honored to help recruit Tom in 1993 to run for the House of Representatives.
“I knew then that he was an intellectual and superb medical doctor. As we worked together, I came to learn he also had the skills that made him arguably the most sought after adviser in the Republican conference. In every policy decision, Tom has sought to be a faithful steward of the taxpayers' money and a dedicated public servant to Oklahoma.”