The Boeing 777-300ER departed from London on Monday and encountered severe turbulence en route. It had to be diverted to Bangkok, where it landed on Tuesday, the airline said in a post on Facebook.
There were 211 passengers and 18 crew on board. The airline extended "its deepest condolences" to the deceased's family.
The airline said it is working with the local authorities in Thailand to provide medical assistance. A team is also on its way to Bangkok.
Details are awaited.