It was the first confirmed sighting of any major wreckage 11 days after Flight 8501 disappeared with 162 people on board.
Powerful currents and murky water continue to hinder the operation, but searchers managed to get a photograph of the debris after it was detected by an Indonesian survey ship, National Search and Rescue chief Henry Bambang Soelistyo told reporters.
One released image appears to show an upside down “A’’ painted on a piece of metal.
The find is particularly important because the all-important cockpit voice and flight data recorders, or black boxes, are located in the aircraft’s tail.
Smaller pieces of the plane, such as seats and an emergency door, had previously been collected from the surface.
Mr Soelistyo said: “Today we successfully discovered the part of the plane that became the main aim since yesterday. I can ensure that this is part of the tail with the AirAsia mark on it.”
He stressed the top priority remains recovering more bodies along with the black boxes. So far, 40 corpses have been found, including an additional one announced yesterday, but time is running out.
At two weeks, most corpses will sink, said Anton Castilani, head of the country’s disaster identification victim unit, and there are already signs of serious decomposition.
Officials are hopeful many of the more than 120 bodies still unaccounted for will be found entombed in the fuselage.
The Airbus A320 went down on December 28, halfway through a two-hour flight between Indonesia’s second-largest city of Indonesia and Singapore, killing everyone on board.
It is not clear what caused the crash, but bad weather is believed to be a contributing factor.
Finding the black boxes will be key to the investigation. They provide essential information including the plane’s vertical and horizontal speeds along with engine temperature and final conversations between the captain and co-pilot.
The ping-emitting beacons still have about 20 days before their batteries go dead, but high surf had prevented the deployment of ships that drag “ping” locators.
Sonar-equipped ships involved in the massive international hunt have also identified what they believe to be the fuselage of the plane. Several other big chunks have been found though no visual confirmation has been received yet.
The search area for bodies and debris was expanded this week to allow for the strong currents that have been pushing debris around, said Indonesian search and rescue operation co-ordinator Tatang Zainudin. In addition to heavy rain and wind, the monsoon weather has made conditions difficult in the Java Sea.