Around 9:30 a.m., a number of visitors to the cape, around 35 km from Tuy Hoa, the capital of the central province, saw the dolphins, each nearly 2.5 m long, swimming in long rows.
Van Duy, a tour guide, said that the pod swam in the area for about 20 minutes.
"I've never seen so many dolphins," he said. "Visitors were delighted to see the dolphins chasing each other in the sea, taking out their phones to record the stunning moment."
Nguyen Trong Thao, a local resident, said schools of dolphins often appeared in the area, known for its calm waves and blue waters.
Three years ago, hundreds of dolphins were spotted near the coast off Phu Yen.
Dai Lanh Cape is a popular tourist attraction in the province. Travelers have to walk along a one-kilometer trail leading to the mountain to get here, where they can then climb 100 wooden steps to reach a lighthouse.