Nhat Phong, 30, a tour guide, said he saw the shark while he and some tourists were going by boat from Ky Co to watch coral reefs at Bai Dua beach.
People in the vessel used their phones to film the shark for 15 minutes.
The whale shark (Rhincodon typus) is a filter-feeding carpet shark and the largest known extant fish species, with the largest confirmed individual being 18.8 meters long.
The fish lives in tropical and warm-temperate seas. It is classified as endangered by the IUCN due to factors including fisheries and vessel strikes.
Earlier this month two whales were seen hunting off the De Gi-Vung Boi coast in Phu Cat District, around 35 kilometers from Quy Nhon.