A statement released by his family said Oberstar died in his sleep. A cause of death was not provided. His former chief of staff, Bill Richard, said Oberstar died at his home in Potomac, Maryland.
Richard said Oberstar was not ill. Oberstar's family said it was heartbroken.
"Jim was a loving husband, father, grandfather, friend and brother," the family statement said. "While we mourn the loss of a good man, we also celebrate his life and his service. We ask for your thoughts and prayers, and understanding, at this very difficult time."
Oberstar, a Democrat, was elected to Congress in 1974 and served 18 terms — the state's longest-serving member of Congress. Oberstar became chairman of the House Transportation and Infrastructure Committee in 2006.
He was a champion for transportation safety and improvements and supported a concept known as intermodality — connecting highways, subways, city buses, intercity rail and bike paths. He also brought jobs to his district, and noted that the economic stimulus brought $212 million to St. Louis County and increased demand for 1.9 million tons of new steel requiring iron ore from the Iron Range's taconite mines.
In 2010, Oberstar narrowly lost to GOP challenger Chip Cravaack as part of a Republican takeover of the U.S. House. After that defeat, Oberstar said: "I go with peace of mind and heart, but with sadness ... I loved the opportunity to serve the people of this district." His district included Duluth and the Iron Range.
"I can't change and wouldn't change any of the votes I cast this year to bring us out of this worst recession, chart a course for the future, to lay a foundation for a better America, a better quality of life, a better quality of health care, rein in financial institutions, to give everybody equal opportunity and a better quality of life," he said after his 2010 loss. "I wouldn't change any one of those things."
Oberstar was the son of an underground miner from Chisholm. The family statement says he was grounded in the hard work, community and family loyalty of the Iron Range region.
Fellow Democrats praised Oberstar on Saturday for his accomplishments and mourned his loss. U.S. Sen. Amy Klobuchar called him a mentor and a man of "true purpose and grit" who traveled the world helping people and speaking his mind, sometimes in French.
"Every miner should remember his work to keep the mines open and make them safer," Klobuchar, also a Democrat, said in a statement. "Every American who bikes the Minnesota bike trails, hikes the Lake Superior trail, and drives our national highways and bridges should remember Jim today."
Gov. Mark Dayton said Oberstar was a true champion for Minnesota residents.
"He worked tirelessly to bring jobs, economic growth, and a better quality of life to his constituents," Dayton said.
After leaving Congress, Oberstar worked as a consultant, and continued to speak publicly. The Humphrey School of Public Affairs at the University of Minnesota created a fellowship in his name. A Great Lakes freighter was also named after him in 2011.
Richard said his death came as a surprise.
"He was active and vivacious and went to one of the grandchildren's plays the night before," Richard said Saturday. "He was going to do more things with the grandchildren today. ... It's a surprise to everyone. I had lunch with him last week and he was in great shape, and alert and physically fit."
Oberstar is survived by his wife, Jean, four children and eight grandchildren.
Associated Press writer Brian Bakst in St. Paul contributed to this report.