The turtle, known to locals as Cu Rua, "great-grandfather turtle" in Vietnamese, was spotted floating lifelessly in Hanoi's Hoan Kiem lake Tuesday afternoon, reports state-run news agency VNS.
Officials say old age and bad weather are to blame for its demise, and that its shell will be preserved at the Vietnam Nature Museum.
The turtle is believed to have been between 80 and 100 years old.
Internet users in Vietnam have been expressing their disappointment on social media since the news broke.
Folk legend in Vietnam has it that Le Loi, a real 15th-century Vietnamese emperor, fended off an attempted Chinese invasion with a magical sword. A mythical turtle was said to have taken that sword from him after his victory.
The lake where this is said to have happened is named after the tale and is known as Ho Hoan Kiem, meaning "lake of the returned sword."
It was in this same lake that Cu Rua lived.
One of a kind
The soft-shell turtle is said to be one of the only four known specimens of the Yangtze giant soft-shell turtle, Rafetus swinhoei.
Cu Rua's demise means that only three are now left in the world, according to the VNS report -- one in another lake in Hanoi and two others in Shanghai, China.
In 2011, it was taken out of the lake and received three months of medical treatment, before it was restored to good health and returned
This trip also uncovered something else.
Unlike what its name suggests, Cu Rua was a female -- not a grandfather at all.
News of the turtle's death came at a politically sensitive time, just one day before the 12th Congress of Communist Party of Vietnam, the country's ruling party, was due to open. This coincidence has not gone unnoticed.