Postoak is an unincorporated community in Jack County, Texas, United States. According to the Handbook of Texas, the community had a population of 79 in 2000.

The area in what is known as Postoak today was established in 1878 and was named for a post oak tree that grew near its main street. A post office was established at Postoak in 1876. It also had a general store, a blacksmith shop, and a church that same year. The community has mostly been a community center and shipping point for local farmers. From the 1930s to the 1970s, Postoak had a little more than 100 residents, which then went down to 79 from 1990 through 2000.

Postoak is located on Farm to Market Road 2127, two miles south of the Clay County line in north-central Jack County.

Today, the community is served by the Midway Independent School District.