The white-thighed surili (Presbytis siamensis) is a species of primate in the Cercopithecidae family. It is endemic to the Thai-Malay Peninsula, the Riau Archipelago and Sumatra. Four subspecies, siamensis (nominate), cana, paenulata and rhionis, are recognized here, but the taxonomy is disputed and in need of a review, and it has included P. natunae as a subspecies, or alternatively both have been considered subspecies of P. femoralis.
