Marys River is a tributary of the Mississippi River in Illinois. It drains a small watershed between the Big Muddy River and the Kaskaskia River. It joins the Mississippi just southeast of Chester, near Kaskaskia. Because of its proximity to Kaskaskia — the capital of Illinois Territory and the first capital of the State of Illinois — Marys River was the site of early settlements leading into the interior of Illinois. Marys River is approximately 41 miles (66 km) in length.