The whale, which was stuck in a fishing net, ran aground near An Bang Island in the Spratly (Truong Sa) archipelago.
Officers of the HQ 641 naval ship rescued the animal and released it into the sea the same day.
The whale weighed an estimated 7-8 tons, the naval officers said.
They said it was the first time they had seen a whale in the island.
In July, a beached dolphin weighing 80 kilograms was rescued by a group of workers near a port in Ha Long City in the northern province of Quang Ninh.
According to local fishermen, there have been many whales, traveling in groups of six to 10, returning to the Ha Long Bay in recent years after a long absence.