Pennsylvania State Police say the teenager was diving off a cliff with friends in southern Lancaster County around 6 p.m. Thursday.
Officials say he never resurfaced.
Dive teams were called in to search the water and canines were searching on the ground.
It happened at a popular gathering spot near the Ferncliff Wildflower and Wildlife Preserve, where the 20- to 30-foot cliffs offer dramatic views of the river.
Crews are expected to resume the search Friday.