Flight QZ8501 vanished from radar screens over the northern Java Sea on Dec. 28, less than half-way into a two-hour flight from Indonesia's second-biggest city of Surabaya to Singapore. There were no survivors among the 162 people on board.
"We've found the tail that has been our main target today," Fransiskus Bambang Soelistyo, the head of the search and rescue agency, told a news conference in Jakarta.
The tail had been identified using an underwater remote operated vehicle, Soelistyo said, adding that the team "now is still desperately trying to locate the black box".
Forty bodies and debris from the plane have been plucked from the surface of the waters off Borneo, but strong winds and high waves have prevented divers from reaching larger pieces of suspected wreckage detected by sonar on the sea floor.
An Indonesian naval patrol ship captain had said on Monday that his vessel had found what could be the tail, but there had been no confirmation of the find.
Locating the tail has been a priority because the cockpit voice and flight data recorders crucial for investigators trying to establish why the plane crashed are located in the rear section of the Airbus A320-200.
"I am led to believe the tail section has been found," AirAsia boss Tony Fernandes tweeted minutes after the announcement.
"If right part of tail section, then the black box should be there ... We need to find all parts soon so we can find all our guests to ease the pain of our families. That still is our priority."
Soelistyo said a total of 12 objects had now been found, but he did not confirm whether all are parts of the aircraft. The wreckage is thought to also include parts of the fuselage, where many of the bodies of victims may still be trapped.
Indonesia AirAsia, 49 percent owned by Fernandes's Malaysia-based AirAsia budget group, has faced criticism from authorities in Jakarta in the 10 days since the crash.
The transport ministry has suspended the carrier's Surabaya-Singapore licence, saying it only had permission to fly the route on Mondays, Tuesdays, Thursdays and Saturdays. Flight QZ8501 took off on a Sunday, though the ministry said this had no bearing on the accident.
AirAsia has said it is cooperating fully with the ministry's investigations. That investigation will be completed by Friday evening, the transport ministry said.
Indonesia has also reassigned some airport and air traffic control officials who allowed the flight to take off and tightened rules on pre-flight procedures in a country with a patchy reputation for air safety.