In a statement, Harkin said that he will be 75 by the time his current term ends and will have been in the House and Senate for 40 years.
"After 40 years, I just feel it's somebody else's turn." he said. "I can't put into words what an honor it is to serve Iowa. And I don't by any means plan to retire completely from public life at the end of this Congress. But I am going to make way for someone new in this Senate seat. I think that is right not just for me, but for Iowa, as well."
Harkin, who chairs the Health, Education, Labor and Pensions Committee, said he has important goals for his remaining two years in the Senate. They include implementing President Obama's health care plan, pushing for a new pension plan, the USA Retirement Plan, and advancing legislation to improve employment for people with disabilities and education.
Born in Iowa, where his father was a coal miner, Harkin attended law school after military service and a stint as a congressional aide. During a trip to Vietnam with U.S. Rep Neal Smith, Harkin photographed and helped publicize the "tiger cage" prisons used by South Vietnam.
After an unsuccessful run for Congress in 1972, Harkin became one of the "Watergate babies," Democrats elected in 1974 after President Richard Nixon's resignation.
President Obama praised Harkin's work in Congress.
"During his tenure, he has fought passionately to improve quality of life for Americans with disabilities and their families, to reform our education system and ensure that every American has access to affordable health care," Obama said.