The man's brother and his teenage son died in the ordeal, according to Russian state media RIA Novosti, who named the survivor as 46-year-old Mikhail Pichugin.
Video of the rescue released by Russian prosecutors shows a bearded man in an orange lifejacket floating on a small catamaran-type vessel with a red flag raised on a pole, as emergency responders work to reach him.
The Sea of Okhotsk is mostly enclosed by Russia's eastern Siberia and the Kamchatka Peninsula. It usually freezes over between October and March, and it ranks as the coldest sea in East Asia.
Two adult men and the 15-year-old son of one of them set off on the catamaran on August 9, prosecutors said.
"After some time, contact with them was lost, their location remained unknown," a spokesperson for Russia's far eastern transport prosecutor's office, Elena Krasnoyarova, said.
"On October 14, around 22:00 the catamaran was spotted by a fishing boat passing in the Sea of Okhotsk near the Ust-Khayryuzovo settlement in the Kamchatka region," she added.
Prosecutors said they are still working to establish the circumstances surrounding the incident and investigating charges of water traffic safety violations, resulting in the death of two or more people through negligence.
The rescued man's wife told Russian state media that his weight could have played a role in his survival, given he weighed about 220lbs (100 kg). She told RIA that Pichugin and his late brother and nephew had enough food to last for about two weeks.
Pichugin will be taken to a hospital for medical treatment in the town of Magadan, in Russia's far east, RIA reported.
He is "in serious condition, emaciated, but conscious," the director of the fishing company that stumbled upon the adrift boat told RIA.
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