The woman was among 32 passengers on board the Ang Thong Explorer when it overturned off Koh Samui today
A passenger list is said to show a number of British tourists on board, as well as German, Romanian and Chinese citizens.
Police said a second woman from Hong Kong also died, while a British man and German woman are missing.
The Foreign Office has confirmed the death and said another British national was being treated for injuries in hospital.
A spokeswoman said: ‘We are supporting the family of a British woman who has sadly died following a boat accident near Koh Samui, Thailand.
‘Another British national has received treatment in hospital for injuries suffered in the same incident. We remain in contact with local authorities in Thailand for further information.’