The U.S. Census Bureau says that the county has a total area of . Of that is land and (1.2%) is water. The lowest point in the state of Kansas is on the Verdigris River in Cherokee Township in Montgomery County (just southeast of Coffeyville), where it flows out of Kansas and into Oklahoma. Western portions of the county contain parts of the northern Cross Timbers eco-region, which separates the forested eastern portion of the United States with the Plains.