Phra That Phu Pek (Thai:พระธาตุภูเพ็ก) is an ancient temple in Sakon Nakhon Province, in the Isan region of Thailand. This ancient Khmer ruin was built from sandstone, standing on a laterite base. This is a Buddhist temple ruin of Khmer origin in the form of a chedi, it was built in the 16th-17th Buddhist century. The ruin is special as it houses a solar calendar, a cube-shaped rock, in its stupa. The ancients used it for indicating the position of the sun, important for religious rites and agricultural seasons.