Yuichiro Miura, who climbed Everest when he was 70 and then again at 75, reached the peak early on Thursday morning, his support team said.
He replaces Nepal's Min Bahadur Sherchan, who was 76 when he conquered Everest in 2008, as the record holder.
But Mr Sherchan, now 81, is set to tackle the mountain again next week.
Mr Miura began his final charge for the 29,035-foot (8,850-meter) peak around 02:00 on Thursday, Japanese media reported, and arrived at the summit some seven hours later.
"I made it!'' Mr Miura said, speaking to his family and supporters via satellite phone from the summit.
"I never imagined I could make it to the top of Mt 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.''
A Nepalese mountaineering official also confirmed to the Associated Press news agency that Mr Miura had made it to the summit.
Mr Miura made the climb with three other Japanese climbers, including his son, and six Nepali Sherpas, Reuters news agency reported.
An extreme skier who once held a world speed-skiing record, Mr Miura broke his pelvis and left thigh in 2009 and has also had a number of operations on his heart.
Ahead of his climb, he said scaling Everest was about challenging his limits and honouring "the great Mother Nature".
"If the limit of age 80 is at the summit of Mt. Everest, the highest place on earth, one can never be happier," he wrote on his expedition website.