In a statement, Ensign said he will send a letter to Vice President Joe Biden Friday indicating his resignation as the 24th U.S. senator from Nevada.
"It is with tremendous sadness that I officially hand over the Senate seat that I have held for 11 years," Ensign said. "While I stand behind my firm belief that I have not violated any law, rule, or standard of conduct of the Senate, and I have fought to prove this publicly, I will not continue to subject my family, my constituents, or the Senate to any further rounds of investigation, depositions, drawn out proceedings, or especially public hearings."
Ensign is facing a Senate Ethics Committee investigation stemming from an affair with Cynthia Hampton, a former aide's wife, in 2009. The aide, Doug Hampton, said Ensign helped him find work as a lobbyist and Ensign's parents gave the Hamptons nearly $100,000.
"I am gratified that, after extended investigations, both the Department of Justice and the Federal Election Commission saw no grounds on which to charge me with improper conduct. I was hopeful that, with the closure of these investigations against me the wear and tear on my family and me would soon be over. This was not the case.
"As is its right, the Senate Ethics Committee is continuing its investigation of issues into which it has been inquiring for the past year and a half. Indeed, the committee even decided recently to devote more resources to its investigation by hiring an outside counsel even though the issues have been viewed and reviewed by so many others. …
"To the people of Nevada, I humbly say thank you for what you have given to me through the years. To my family, thank you for the support and love that you have shown me. To my staff, thank you for coming on this incredible journey with me and for standing by me despite the obstacles."
U.S. Reps. Dean Heller, R-Nev., and Shelley Berkley, D-Nev., are widely expected to run for Ensign's seat.
Notable deaths of 2014 [PHOTOS]