Can you camp overnight at White River Campground once it closes for the season?

No. Once White River Campground closes for the season, overnight camping there is not permitted.

Mount Rainier National Park's campground status page lists White River Campground as closed for the season, noting that only day use parking remains open and that restrooms are available solely in Loop B. Source: https://www.nps.gov/mora/planyourvisit/campgrounds.htm

The park's broader camping rules reinforce that overnight stays are confined to open, designated campsites: "Camp in designated campsites only. Sleeping in vehicles outside of campgrounds is not permitted." Source: https://www.nps.gov/thingstodo/camping-at-mount-rainier.htm

White River Campground's typical operating season runs from late June to late September, and the park reminds visitors to check the Operating Hours page between seasons to confirm current status before planning a trip. Source: https://www.nps.gov/mora/planyourvisit/campgrounds.htm

Anyone wanting to camp overnight near White River after the campground itself closes would need a backcountry wilderness permit rather than relying on the closed campground, since overnight camping in the park's wilderness areas requires a permit obtained through Recreation.gov or a park Wilderness Information Center. Source: https://www.nps.gov/mora/planyourvisit/wilderness-camping-and-hiking.htm

Because the campground is officially closed, there are no first-come, first-served sites, no fee collection, and no staffed facilities to support overnight stays. Visitors should plan to use the campground only during its posted operating season and shift to a reserved wilderness permit or another open campground, such as Cougar Rock, once White River has closed for the year.