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.

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.

Phra That Phu Pek is in tambon Na Hua Bo, Phanna Nikhom district, Sakon Nakhon Province of northeastern Thailand, about 37 km west of the town of Sakon Nakhon.