Vassar is an unincorporated community in Osage County, Kansas, United States. Vassar is located along a railroad line 4 miles (6.4 km) northeast of Lyndon. Vassar has a post office with ZIP code 66543.
A post office was opened in La Mont's Hill (an extinct town) in 1871, but it was moved to Vassar in 1887.
The climate in this area is characterized by hot, humid summers and generally mild to cool winters. According to the Köppen Climate Classification system, Vassar has a humid subtropical climate, abbreviated "Cfa" on climate maps.