Nirmal Purja, 36, known as Nims, broke the previous record of a South Korean climber, Kim Chang-ho, who achieved the feat in 2003 to scale all the peaks higher than 8,000 metres in seven years, 10 months and six days.
Mingma Sherpa, chairman of Seven Summit Treks, the agency handling Purja's climb, told Efe news that the Nepali mountaineer reached the Mt Shishapangma summit (8,027 metres) at 8.58 a.m.
He said three other climbers accompanied Purja, who is known for his viral May 25 photograph of a traffic jam of climbers leading to the summit of Mt. Everest.
Purja and his team set out on the mission, "Project Possible 14/7", on April 23 to summit all 14 of the 8,000s and started by climbing mountains from Annapurna (8,091 metres).
"He completed his mission in 190 days today," said Mingma Sherpa.
Purja, a former soldier with United Kingdom Royal Navy's elite Special Boat Service, is aiming to raise nearly $1 million. He has set up a GoFundMe campaign and has vowed to use the money for children in need in Nepal.
He also posted his "mission achieved" messages on his social media pages after scaling Shishapangma. The peak was the last in his list.
Kim Chang-ho climbed all 8,000s without using supplemental oxygen. He was killed in a snowstorm at the base camp of Mt Gurja in Nepal's Myagdi district in October 2018.
Polish Jerzy Kukuczka is the first mountaineer to climb all 14 peaks, a feat he achieved in seven years, 11 months and 14 days in 1987.
Chinese authorities had granted Purja and his team a special permit to scale Shishapangma at the request of the Nepali government.
Purja left Nepal for Tibet on October 18, leading a five-member expedition to climb the peak.
In May, Purja climbed Everest (8,848 metres), descended to South Col and climbed adjoining Lhotse (8,516 metres), then flew to Makalu base camp and climbed that peak (8,481 metres) too, all in 48 hours.
He broke his previous record for the same three peaks, taking five days in 2017.
( With inputs from IANS )