Eureka Township is a charter township of Montcalm County in the U.S. state of Michigan. The population was 4,211 at the 2020 census, up from 3,959 in 2010. The township surrounds the city of Greenville, a separate municipality, and the Greenville post office, with ZIP code 48838, serves all of Eureka Township. Eureka Township was established in 1850.