An AirAsia airliner flying from Indonesia to Singapore with 162 people on board has lost contact with air traffic control.
Flight QZ8501, an Airbus plane, went missing at 07:24 (23:24 GMT), Malaysia-based AirAsia tweeted.
Indonesian military planes are searching an area of the Java Sea.
AirAsia, a budget airline, has never lost a plane, but Malaysia's national carrier Malaysia Airlines has suffered two this year - flights MH370 and MH17.
The AirAsia plane disappeared about two hours into a three-hour flight.
It left the Indonesian city of Surabaya in eastern Java at 05:20 local time (21:20 GMT) and was due to arrive in Singapore at 08:30 (00:30 GMT).
The missing jet had requested a "deviation" from the flight path due to bad weather, the company said.
Indonesia's transport ministry said the pilot had asked permission to climb to 38,000 ft (11,000m) to avoid thick cloud. No distress call is reported to have been issued by the crew.
An official with the transport ministry, Hadi Mustofa, told local media the plane lost contact over the Java Sea, between the islands of Kalimantan and Java.
The company's chief executive, Tony Fernandes, tweeted: "Thank you for all your thoughts and prayers. We must stay strong."
AirAsia Indonesia is an affiliate of the Malaysian company AirAsia, operating domestic flights round the Indonesian archipelago as well as international services to Malaysia, Singapore, Australia and Thailand.
AirAsia Indonesia was banned from flying to the European Union in 2007 due to safety concerns but this was lifted in July 2010.
The latest incident caps a tragic year for Malaysian aviation with the disappearance of one Malaysia Airlines plane and the apparent shooting down of another.
Flight MH370 disappeared on a flight from Kuala Lumpur to Beijing in March with 239 passengers and crew. The wreckage, thought to be in southern Indian Ocean, has still not been located.
MH17 was shot down over Ukraine in July, killing all 298 on board.