Yuichiro Miura, who also conquered the 29,035-foot (8,850-meter) peak when he was 70 and 75, reached the summit at 9:05 a.m. local time, according to a Nepalese mountaineering official and Miura's Tokyo-based support team.
Miura and his son Gota made a phone call from the summit, prompting his daughter Emili to smile broadly and clap her hands in footage shown by Japanese public broadcaster NHK.
"I made it!" Miura said over the phone. "I never imagined I could make it to the top of Mount Everest at age 80. This is the world's best feeling, although I'm totally exhausted. Even at 80, I can still do quite well."
The climbers were going to take pictures at the summit before starting to descend, Miura's office said.
Miura conquered the mountain despite undergoing heart surgery in January for an irregular heartbeat, or arrhythmia, his fourth heart operation since 2007, according to his daughter. He also broke his pelvis and left thigh bone in a 2009 skiing accident.
Nepalese mountaineering official Gyanendra Shrestha, at the Everest base camp, confirmed that Miura had reached the summit and was the oldest person to do so.
The previous oldest was Nepal's Min Bahadur Sherchan, who accomplished the feat at age 76 in 2008, just a day before Miura reached the top at age 75.
Sherchan, now 81, was preparing to scale the peak next week despite digestive problems he suffered several days ago. On Wednesday, Sherchan said by telephone from the base camp that he was in good health and "ready to take up the challenge."
Sherchan's team leader, Temba, who uses one name, said that he would congratulate the new record holder once he returned to the base camp and that Sherchan would not turn back until he completes his mission.
Sherchan also got good news Thursday when Nepal's government approved financial aid for his climb.
The Cabinet approved 1 million rupees ($11,200) in aid for Sherchan's expedition and also waived the $70,000 permit fees, said Bimal Gautam, the press adviser to the Chairman of the Council of Ministers.
On his expedition's website, Miura explained his attempt to scale Everest at such an advanced age: "It is to challenge (my) own ultimate limit. It is to honor the great Mother Nature."
He said a successful climb would raise the bar for what is possible.
"And if the limit of age 80 is at the summit of Mount Everest, the highest place on earth, one can never be happier," he said.
Miura became famous when he was a young man as a daredevil speed skier.
He skied down Everest's South Col in 1970, using a parachute to brake his descent. The feat was captured in the Oscar-winning 1975 documentary, "The Man Who Skied Down Everest." He has also skied down Mount Fuji.
It wasn't until Miura was 70, however, that he first climbed to the top of Everest. When he summited again at 75, he claimed to be the only man to accomplish the feat twice in his 70s. After that, he said he was determined to climb again at age 80.
Associated Press writers Malcolm Foster and Mari Yamaguchi contributed to this report from Tokyo.
Copyright 2013 The Associated Press.