Lion Air flight JT 610 has come down shortly after take-off from Indonesian capital, Jakarta.
Some 188 people were on board the Boeing 737 as it crashed into the sea on route to Pangkal Pinang, in the Bangka Belitung Islands.
It lost contact with air traffic controllers after take-off and is believed to have ended up in the water.
Authorities are on the scene, with no survivors currently reported.
“We do not know yet whether there are any survivors,” Indonesia Search & Rescue Agency chief, Muhmmad Syaugi, told reporters.
The flight took-off from Soekarno-Hatta airport in Jakarta at 06:20 local time on Monday morning, before asking to return to the airfield.
It was due to arrive at Depati Amir airport in Pangkal Pinang an hour later but 13 minutes into the flight, authorities lost contact with the plane.
The aircraft was a Boeing 737 MAX 8, a model only in commercial use since 2016.
It is a single aisle plane used for short-haul travel.
Edward Sirait, chief executive of Lion Air Group, told Reuters: “We don’t dare to say what the facts are, or are not, yet.
“We are also confused about the why, since it was a new plane.”