William O'Brien State Park is a 1,520-acre state park of Minnesota, USA, along the St. Croix River. Its hiking trails traverse rolling glacial moraine, riparian zones, restored oak savanna, wooded areas and bogs. It is a popular place for birdwatching, picnics, camping, cross-country skiing, canoeing, fishing, and other typical outdoor recreational activities. The park is located approximately two miles north of the town of Marine on St. Croix, Minnesota.