Ouachita Parish is a parish located in the northern part of the U.S. state of Louisiana. As of the 2020 census, the population was 160,368. The parish seat and largest city is Monroe. The parish was formed in 1807. Ouachita Parish is part of the Monroe, Louisiana Metropolitan Statistical Area. Located here is Watson Brake, the oldest indigenous earthwork mound complex in North America. It was built around 3500 BCE, making it older than the Ancient Egyptian pyramids or Britain's Stonehenge.