A Tigerair flight heading to Singapore was diverted to Bangkok on Tuesday after a Singaporean passenger died on board.
The 58-year-old was travelling from Chiang Mai with his Thai wife and several other family members, local Thai media reported.
The wife told Thai airport officials that her husband was suffering from a chronic illness, although it was not made clear what that was.
The man is believed to have fallen ill during the flight, which forced the pilot to seek an emergency landing at Bangkok's Suvarnabhumi Airport for medical help. But he died on the plane.
According to reports, his body was taken to the Police General Hospital for an autopsy.
Tigerair's spokesman told The Straits Times: "Our immediate concern is for these next of kin and we are doing what we can to assist them."
Flight TR2177, which left Bangkok with the rest of the passengers, continued on its journey and arrived safely at Changi Airport that same evening.
Deaths during flights are rare but do occur, airline cabin crew told The Straits Times. In a medical emergency, the procedure is to check whether there is a doctor on board so the sick passenger can be attended to.
Depending on how serious the situation is, the pilot can continue with the flight or divert the plane to the nearest airport to get medical help.
Said a cabin crew member: "If a passenger is visibly unwell before take-off, we do advise him or her to stay behind, but it's not always easy because people don't want to disrupt their travel plans.
"In such cases, we would call in a doctor to certify the passenger is fit to travel."