Big Oak Tree State Park is a state-owned nature preserve with recreational features encompassing 1,029 acres (416 ha) in East Prairie, Missouri, United States. The state park was established in a large expanse of drained cropland in 1938 to protect some of the largest trees in the state and in the nation. The park was declared a National Natural Landmark in May 1986, recognized as a rare, untouched wet-mesic bottomland hardwood forest in the Mississippi Alluvial Plain portion of the Gulf Coastal Plain.