Little is known about mating behaviors in arboreal snakes. We report three observations of mating in wild big-eyed pit vipers (Trimeresurus macrops) in northeast Thailand from 2013 to 2015. Mating occurred at the end of the rainy season, between 20 September and 28 October. One copulation occurred on the ground and two were arboreal (<1.5 m high). Females pulled males up trees during arboreal mating. Observations lasted from 2.2 to 12.9 h, but two of the three observations were initiated after copulation had started and were thus incomplete. Females were larger than males in all copulations. On one occasion a second male attempted to disrupt a mating pair using aggressive body movements and tail wrestling. All mating occurred in human disturbed forests, and two of them were observed in close proximity to human habitations.