Stevens is the leader of the liberal bloc on the high court, but his retirement is not expected to change the philosophical structure of the court -- four liberals, four conservatives and moderate conservative Justice Anthony Kennedy.
Obama is expected to nominate a liberal, possibly U.S. Solicitor General Elena Kagan, as a successor, but the following confirmation process in the Senate could unleash a lengthy fight with Republicans still smarting from the healthcare battle.
Stevens sent a "Dear Mr. President" letter to Obama:
"Having concluded that it would be in the best interests of the court to have my successor appointed and confirmed well in advance of the commencement of the court's next term" in October, "I shall retire from regular active service as an associate justice, under the provisions (federal law), effective the next day after the court rises for the summer recess this year."
The "rising" of the court for the summer recess has no fixed date. It occurs when the court's business is done for the term, traditionally in late June.
If Stevens, who turns 90 April 20, had served for two more years, he would have surpassed the late Justice William O. Douglas for the longest tenure on the court. Douglas served 36 years and 209 days before retiring in 1975.
Stevens is close to equaling the late Justice Oliver Wendell Holmes as the oldest serving justice. Holmes retired in 1932 after his 90th birthday.
Stevens was appointed by Republican President Gerald Ford in 1975.