Phu Soi Dao, Mountain peak at Thailand-Laos border.

Phu Soi Dao is a 2,120-metre mountain at the southern end of the Luang Prabang Range, on the Laos-Thailand border. Its forest holds Pinus kesiya, Betula alnoides, Schima wallichii and Shorea siamensis, and the range area contains numerous waterfalls. The Thai side lies in Phitsanulok and Uttaradit Provinces, within Phu Soi Dao National Park.

Phu Soi Dao rises at the southern end of the Luang Prabang Range, and the crest of the range forms the border between Laos and Thailand. The Thai side, in Phitsanulok and Uttaradit Provinces, was declared Phu Soi Dao National Park. The sources record no border disputes or territorial negotiations connected with the mountain.

The name Phu Soi Dao combines the Lao and Thai words for mountain, phu, with Soi Dao; the French form Phou Soai Dao appears on older maps. The Thai side lies within Phu Soi Dao National Park, spanning Phitsanulok and Uttaradit Provinces, where the mountain's forests and waterfalls are protected. Two bladderworts, Utricularia spinomarginata and Utricularia phusoidaoensis, are known only from this mountain.

To use the hiking trails, visitors need permission from the park authorities, which should be obtained before arriving. The period from December to February offers the most favorable climbing conditions, with drier weather and better visibility.

Phu Soi Dao is one of the few places where the bladderworts Utricularia spinomarginata and Utricularia phusoidaoensis grow, both described as endemic to this mountain in the Luang Prabang Range. The forest around them also holds Pinus kesiya, Betula alnoides, Schima wallichii and Shorea siamensis.