Posey Island is a one-acre marine state park located north of Pearl Island near Roche Harbor, accessible only by human-powered watercraft (kayaks, canoes, paddleboards).

The island features two primitive group campsites (max 16 people per night) with a composting toilet, fire rings, and picnic tables; camping reservations are required and available up to nine months in advance through the Cascadia Marine Trail.

Access is via a sandy spit on the south side that varies with tides, with the nearest motorized boat launch at Roche Harbor Resort.