Coldwater is a city in and county seat of Branch County in the U.S. state of Michigan. The city had a population of 13,822 at the 2020 census. Th…Coldwater is a city in and county seat of Branch County in the U.S. state of Michigan. The city had a population of 13,822 at the 2020 census. The city is mostly surrounded by Coldwater Township, but the two are administered autonomously.