The tail section, marked with part of the airline's logo, was detected by sonar early Wednesday, said Bambang Soelistyo, the head of Indonesia's search and rescue agency.
Divers were sent to the location and plunged into the waves. Down on the sea floor, they were able to take pictures of the wreckage.
Finding the tail section is a potentially vital step in the investigation because that is where the flight data and voice recorders -- the so-called black boxes -- are located in the Airbus A320-200, the aircraft model of Flight QZ8501.
"The cockpit voice recorder and flight data recorder are absolutely crucial to gaining an understanding of what happened to this aircraft," said Greg Waldron, the managing editor of Flightglobal, an aviation industry website.
Are the black boxes there?
But it's still uncertain if the black boxes are in the piece of wreckage the searchers discovered.
"I am led to believe the tail section has been found," tweeted AirAsia Group CEO Tony Fernandes. "If right part of tail section then the black box should be there."
That's assuming the flight recorders didn't come loose as the plane went down and hit the water.
"It's by no means certain that the recorders are located within the tail," Waldron said, noting that searchers haven't reported detecting any pings from the black boxes' locator beacons.
"They might have fallen free, which could create some issues," he said.
The recorders are generally considered the key piece of evidence when it comes to investigating a commercial plane disaster.
The data recorder provides a wide range of valuable information about what the plane was doing, from its air speed to the position of the landing gear.
The cockpit voice recorder captures communications between the pilots. In this case, officials hope it will contain any possible discussion of the crisis that eventually brought down the jet, Waldron said.
Investigators will be looking for answers why Flight QZ8501 dropped off radar and went down December 28. The plane lost contact after the pilot requested permission to turn and climb to a higher altitude, according to Indonesian officials.
Divers to keep looking
Soelistyo said Wednesday that divers were preparing to go back underwater in the same area, which is in one of the priority zones where search efforts have been focused. He didn't say at what depth the tail section had been found.
"Elements are also still conducive for another search. They also are striving to find the black box," Soelistyo told Indonesia's Channel tvOne.
One of the images released by the search agency showed what appeared to be the letters A and X; another showed the word "Air" from the AirAsia logo.
Searchers have been scouring the choppy waters of the Java Sea for remains from the commercial jet since it lost contact, with 162 people on board. It was flying from the Indonesian city of Surabaya to Singapore.
Search teams had located several large pieces of wreckage believed to be from the plane but none of them as significant as the tail.
Difficult conditions -- including thunderstorms, strong winds and big waves -- have hampered the search efforts, which are now in their 11th day.
Soelistyo also said Wednesday that another body had been found in the sea, bringing the total to 40. Some of the bodies recovered previously have been discovered still strapped into seats.
Many family members of the people on board the plane have been waiting in Surabaya for news of their loved ones. Of the 40 bodies discovered, 16 have been identified publicly.
The vast majority of the people on the plane were Indonesian. There were also citizens of Britain, France, Malaysia, Singapore and South Korea.
The airline said Wednesday that it will offer family members roughly $100,000 in compensation per passenger. The amount is in addition to $24,000 offered to families to help with immediate financial hardship.