Forestry officials say a survey, using hidden cameras, counted 106 tigers in the wild on the Bangladeshi side of the world's largest mangrove swamp, and about 74 on the Indian side.
The figures mark a sharp decline from the 440 animals recorded 10 years ago.
Experts say the decline is mainly down to rampant poaching.
But they add that lower numbers also reflect more accurate surveying techniques.
All previous surveys counted tiger paw prints - which are unique to each animal, but difficult to spot, collate and analyse.
The hidden cameras yielded lower - but more accurate figures, they say.
Experts say the gangs killing the tigers trade in tiger skin and body parts.
"The numbers have declined largely because of poaching, which is the main threat to the tigers in Sunderbans," Dr Anwarul Islam, Professor of Zoology at Dhaka University, told the BBC.
"The threat comes not only from stray poaching, which is rampant, but  also from organised gangs of poachers.
"Unless we have an independent, dedicated anti-poaching unit, the future is not bright for the tigers in Bangladesh."
There are now fewer than 2,300 Bengal tigers left in the wild - mainly in India and Bangladesh, but with smaller populations in Nepal, Bhutan, China and Myanmar (also called Burma).
The Sunderbans is made up of 10,000 square kilometres of dense forest straddling India and Bangladesh.
Bangladesh has only a little more than 100 Royal Bengal tigers living in the Sunderbans forest, far fewer than previously thought, new figures show.