Three Island Crossing State Park, Historic state park in Glenns Ferry, Idaho, US.

Three Island Crossing State Park is a state park along the Snake River near Glenns Ferry, in Elmore County, Idaho. The park sits at the spot where three islands once divided the river, giving settlers a route to cross on their way west along the Oregon Trail.

The three islands made this the most-used ford on the Oregon Trail, until Gus Glenn opened a ferry about 2 miles (3.2 km) upstream in 1869. The crossing declined after that, and the land passed from the city of Glenns Ferry to the Idaho Department of Parks and Recreation in 1968. It opened as a state park in 1971.

The visitor center has exhibits about the encounters between pioneers and Native American communities along the Oregon Trail. Walking through the displays gives a clear sense of why this river crossing mattered so much to those heading west.

The park is open year-round and the trails are well marked, making it easy to get around without prior experience. Visitors coming in summer should bring plenty of water, since much of the terrain is exposed to direct sun.

The park's name comes from three islands that once split the Snake River's current, allowing wagons to ford in stages. Gus Glenn later ran a ferry about 2 miles upstream, and the crossing fell out of use after 1869. Today the park offers camping, cabins, disc golf and a visitor center with interpretive exhibits.