Local police said Monday that it was the second death in the park after a four-year-old boy drowned in June, just three days after the park opened.
The unidentified six-year-old boy had gone with his relatives to the Thanh Ha Water Park on Sunday when he fell into an artificial river, local media reported.
He was given first aid and taken to hospital, but he died on Monday.
The park has put up a sign saying it has temporarily suspended operations as of Monday afternoon.
An investigation is ongoing.
Drowning is one of the main causes of child deaths in Vietnam. According to a recent survey by the World Health Organization, more than 11,000 children drown each year in the country.