Singapore Airlines did not say at what point the injuries and death took place, but a passenger who spoke to Reuters said turbulence led to those not wearing seatbelts hitting overhead cabins.
Thai media reports said there were 30 injuries, while Singapore Airlines did not specify how many people were injured.
The Boeing 777-300ER plane with 211 passengers and 18 crew was headed to Singapore when it made the emergency landing, the airline said in a statement.
After about 11 hours of flying time from in London, the aircraft on Tuesday sharply dropped from an altitude of about 11,300 metres to 9400m within just five minutes as it finished traversing the Andaman Sea and neared Thailand, according to FlightRadar 24 data.
"Suddenly the aircraft starts tilting up and there was shaking so I started bracing for what was happening, and very suddenly there was a very dramatic drop so everyone seated and not wearing seatbelt was launched immediately into the ceiling," Dzafran Azmir, a 28-year-old student on board the flight told Reuters.
"Some people hit their heads on the baggage cabins overhead and dented it, they hit the places where lights and masks are and broke straight through it," he said.
Bangkok's Suvarnabhumi airport official confirmed one death but could not confirm the total injured.
Thai immigration police said medical personnel had boarded the plane to assess injuries but could not confirm the number.
It said uninjured passengers had disembarked.
"Our priority is to provide all possible assistance to all passengers and crew on board the aircraft," the airline said.
"We are working with the local authorities in Thailand to provide the necessary medical assistance."
Turbulence-related airline accidents are the most common type, according to a 2021 study by the National Transportation Safety Board.
From 2009 through 2018, the US agency found that turbulence accounted for more than a third of reported airline accidents and most resulted in one or more serious injuries, but no aircraft damage.
Singapore Airlines, which is widely recognised as one of world's leading airlines and is a benchmark for much of the industry, has not had any major incidents in recent years.
Its last accident resulting in casualties was a flight from Singapore to Los Angeles via Taipei, where it crashed on October 31, 2000, into construction equipment on the Taiwan Taoyuan International Airport after attempting to take off from the wrong runway.
The crash killed 83 of the 179 people on board.
Singapore Airlines has had seven accidents according to records by the Aviation Safety Network.
Boeing did not immediately respond to a request for comment.
