"Singapore Airlines offers its deepest condolences to the family of the deceased. We deeply apologise for the traumatic experience that our passengers and crew members suffered on this flight," the airline said, adding it was working with Thai authorities to provide all necessary assistance.
The flight fell into an air pocket while cabin crew was serving breakfast before it encountered turbulence, prompting the pilots to request an emergency landing, Suvarnabhumi airport general manager Kittipong Kittikachorn told a press conference.
A 73-year-old man from the United Kingdom died during the incident, likely due to a heart attack, he said.
Seven people were critically injured with head injuries.
Eighteen people have been hospitalised, Singapore Airlines said.
Singapore Airlines says 18 people were treated in hospital after their plane landed in Bangkok. (AP PHOTO)
Tracking data captured by FlightRadar24 and analysed by the Associated Press show the Singapore Airlines flight SQ321 cruising at an altitude of 11,300 metres.
At one point, the Boeing 777-300ER suddenly and sharply plummets to 9400 metres over the span of three minutes, the data shows.
The aircraft then stayed at 9400 metres for under 10 minutes before diverting and landing in Bangkok less than half an hour later.
The sharp descent in turbulence happened as the flight was over the Andaman Sea, near Myanmar.
The aircraft sent a "squawk code" of 7700 at that time, an international emergency signal.
A passenger who was on the flight told Reuters that the incident involved the sensation of rising then falling.
"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 a 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.
Kittikachorn said most of the passengers he had spoken to had been wearing their seatbelts.
The plane with 211 passengers and 18 crew was headed to Singapore when it made the emergency landing, the airline said.
Singapore news outlet CNA carried blurry pictures supplied by readers that it said appeared to be from the flight.
They showed anxious passengers clinging to seats, with oxygen masks hanging from above, personal items strewn across the aisle and rubbish spilled on the floor of the cabin crew area.
Suvarnabhumi airport said the plane requested an emergency landing at 3.35pm local time and landed at 3.51pm.
Uninjured passengers disembarked and an another aircraft will fly them onwards.
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 said it was in touch with Singapore Airlines and was ready to provide support.
It referred further questions to the airline and local authorities.
"We extend our deepest condolences to the family who lost a loved one, and our thoughts are with the passengers and crew," it said.
with AP
