Porlajee Rakchongcharoen, also known as Billy, has not been seen since April 17 in Kaeng Krachan National Park in Phetchaburi.
"We are concerned about the lack of progress with an investigation into the disappearance of a prominent human rights defender in Thailand," UN human rights agency spokesman Rubert Colville said in Geneva.
"This case illustrates a disturbing pattern of killings and disappearances of environmental activists in Southeast Asia."
Park authorities arrested the Karen activist on April 17 for possessing a wild honeycomb and six bottles of wild honey but released him after giving a warning. He disappeared shortly afterward. Mr Porlajee led a Karen group that filed a complaint in 2012 with the Central Administrative Court against the Department of National Parks, Wildlife and Plant Conservation and the Natural Resources and Environment Ministry.
The case focuses on the actions of Kaeng Krachan National Park chief Chaiwat Limlikitaksorn and other forestry officials, who burned down 20 Karen houses in a 2011 operation to deal with forest encroachment. Mr Porlajee had been collecting evidence and interviewing witnesses to press his case.
Mr Chaiwat has denied any involvement in the disappearance of the activist.
The Lawyers Council of Thailand helped Mr Porlajee's wife, Pinnapa Prueksapan, to petition the Phetchaburi Court for an urgent hearing on her husband's disappearance.
The only access to Billy's village, Bangkloybon, is from the national park in Kaeng Krachan district.