Hayden Klemenok was last seen backpacking with a group at Upper Chilnualna Fall on Sunday afternoon.
Park officials say he entered Chilnualna Creek near the trail junction at about 2 pm but hasn't been seen since.
He was last seen wearing a red t-shirt, blue swimming trunks and white sneakers with a tan hat and sunglasses.
If you see him, contact Yosemite National Park dispatch.