The park protects 65.93 km², mostly on the island Pha Ngan. The park was established in 1983 covering an area of 39.12 km², and was enlarged to its present size in 1999.
The park is named after the river Than Sadet (Royal River). The river forms the largest waterfall on the island, which was visited by several Thai kings. Especially Rama V liked the place and visited 14 times. Not far from the waterfall the river empties into the sea at Than Sadet Bay, with its sandy beach another popular place within the park.
Khao Ra is both the highest elevation of the park as well as on the whole island of Pha Ngan, peaking at 727 m above sea level.
In addition to the forest on Pha Ngan itself, the park also includes several small islands north of Pha Ngan, like the Wao and Hin Bai archipelagos.