In a Facebook post on Monday, the Khlong Lan National Park congratulated diver Paek Khayok for finding the one-karat diamond ring for Bangkok resident Nit Phokul.
Nit was in the waterfall last Thursday when she slipped and fell, and the water rapids pulled the ring off her finger.
Two days later, Surachai Pokamanee, chief of the Khlong Lan development office, called on treasure hunter Paek to help.
Though the area where Nit had lost her ring was full of slippery rocks and rapidly flows, Paek managed to locate the gem in 35 minutes using his metal detector.
He then ceremoniously handed the ring over to a smiling Nit, who slipped it on and posed for a selfie with him.