Khao Yai National Park is a national park in Thailand. It lies largely in Nakhon Ratchasima province (Khorat), but also includes parts of Saraburi, Prachinburi and Nakhon Nayok provinces. It was the country's first national park, established on September 18 1962.

The park is the second largest in Thailand. It covers an area of 2,168 square kilometers, including evergreen forests and grasslands. Its altitude mostly ranges from 400m to 1000m above sea level. There are 3,000 species of plants, 320 species of birds and 67 species of mammals, including elephants, tigers, gibbons, deer and wild pigs. Its waterfalls include the 80 metre Haew Narok.

The park is often visited by travellers from Isan, Bangkok and beyond, for whom there are over 50km of hiking trails. Since 1991 the park itself has been closed to overnight visitors.