The 57-year-old Dowiyogo died Sunday afternoon at the George Washington University Hospital, spokeswoman Helen Bogden said.
Dowiyogo underwent an 11-hour heart operation Tuesday.
Bogden said the U.S. Air Force had agreed to fly Dowiyogo's body back to Nauru later in the week for a funeral service.
A diabetic for 30 years, Dowiyogo underwent coronary artery bypass surgery in Melbourne in 2001 and required alternate-day dialysis.
His health deteriorated rapidly after his return to the presidency in January. About a month ago, he underwent an emergency operation in Nauru by an Australian surgeon visiting the island.
Nauru, situated about halfway between Australia and Hawaii, has a population of about 12,300 on an island of 8 square miles.
Dowiyogo first became an elected member of Nauru's 18-seat parliament in 1973. He served his first term as president in 1976-1978 after ousting the island's head chief and first president, Hammer DeRoburt, who helped Nauru win its independence from Australia in 1968.
Dowiyogo served as president in 1989-1995, for 15 days in 1996, and in 1998-1999 and 2000-2001 before again taking office in January.
Dowiyogo is survived by a wife and four children.
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