Authorities said the man, reported to be in his 30s but not named, sustained serious injuries to his arm and face. He remained conscious and was airlifted to a local hospital.
According to local media, emergency services received a call reporting the incident at around noon on Sunday.
The man had been hiking alone when he came across the bear. He was not carrying a bear bell or other deterrent.
In the wake of the attack, police and local hunters searched the area but could not find the bear, Kyodo News reported.
Authorities later installed a cage in the town in a bid to capture the animal.
Okutama is popular with hikers and located less than an hour's drive outside Tokyo's western suburbs.
The area has reported roughly 40 bear sightings since April, officials noted.
Japan confirmed its first fatal bear attack of 2026 earlier this month. The East Asian nation saw an unprecedented 13 casualties last year.
Authorities said a 55-year-old woman found dead in the northern Iwate prefecture last month was killed by a bear. Two other bodies discovered in Iwate on 7 May and Yamagata prefecture on 5 May, respectively, were suspected to be linked to bear attacks as well.
One of them was identified by NHK as 69-year-old Chiyoko Kumagai, who had disappeared after going into a mountain forest to collect edible wild plants.
Japan saw a surge in dangerous bear encounters in 2025, with the animals entering homes, schools, supermarkets, and hot spring resorts.
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