The Tugen Hills (also known as Saimo) are a series of hills in Baringo County, Kenya. They are located in the central-western portion of Kenya.

The Tugen Hills represent one of the few areas in Africa preserving a succession of deposits from the period of between 14 and 4 million years ago, making them an important location for the study of human (and animal) evolution. Excavations at the site conducted by Richard Leakey and others have yielded a complete skeleton of a 1.5-million-year-old elephant (1967), a new species of monkey (1969) and fossil remains of hominids from 1 to 2 million years ago. In 1974 Martin Pickford found a singular fossilised molar of a Orrorin tugenensis there, and that encouraged him to return 30 years later. In 1975, he named the fossilised finds Orrorin tugenensis, which means: “Original man of Tugen Hills”. This hominid lived from 6.2 MYA to 5.6 MYA.

Six-million-year-old hominid fossils were discovered here in 2000 by Brigitte Senut and Martin Pickford; the species was named Orrorin tugenensis after the location. This was the oldest hominid ever discovered in Kenya, and the second oldest in the world after Sahelanthropus tchadensis.