Harkin, 73, is one of the most liberal senators and has focused much of his long congressional career on farm policy and legislation aiding people with disabilities.
"I don't by any means plan to retire completely from public life at the end of this Congress," Harkin said in a statement. "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."
In his remaining two years in office, Harkin said he will focus on implementing President Barack Obama's healthcare reforms, expanding job opportunities for people with disabilities and expanding access to education.
"When the current Congress is over, I will have served in the United States House of Representatives and the U.S.Senate for a total of 40 years. After 40 years, I just feel it's somebody else's turn," he said.