Cass County is a county in the U.S. state of Nebraska. As of the 2020 United States Census, the population was 26,598. Its county seat and largest city is Plattsmouth. The county was formed in 1855, and was named for General Lewis Cass, an American military officer, politician, and statesman.

Lewis Cass was an American military officer, politician, and statesman. He represented Michigan in the United States Senate and served in the Cabinets of two U.S. Presidents, Andrew Jackson and James Buchanan. He was also the 1848 Democratic presidential nominee. A slave owner himself, he was a leading spokesman for the doctrine of popular sovereignty, which at the time held the idea that people in each U.S state should have the right to decide on whether to permit or prohibit slavery, believing in the idea of states rights.