Phu Khao Thong (Golden mountain, ภูเขาทอง) is a steep artificial hill inside the Wat Saket compound.
Rama I's grandson, King Rama III (1787–1851), decided to build a chedi of huge dimensions inside Wat Saket, but the chedi collapsed during construction because the soft soil of Bangkok could not support the weight. Over the next few decades, the abandoned mud-and-brick structure acquired the shape of a natural hill and was overgrown with weeds. The locals called it the "phu khao" (ภูเขา), as if it were a natural feature.
During the reign of King Rama IV, construction began of a small chedi on the hill. It was completed early in the reign of his son, King Rama V (1853–1910). A relic of the Buddha was brought from Sri Lanka and placed in the chedi. The surrounding concrete walls were added in the 1940s to stop the hill from eroding. The modern Wat Saket was built in the early 20th century of Carrara marble.
An annual festival is held at Wat Saket every November, featuring a candlelight procession up Phu Khao Thong to the chedi.
Phu Khao Thong is now a popular Bangkok tourist attraction and has become one of the symbols of the city.