Naw Kham, an ethnic Shan warlord, is widely believed to run much of the smuggling and narcotics business in the so-called Golden Triangle region, where the borders of Laos, Burma and Thailand meet.
He was detained in April in Laos and extradited in May to China as chief suspect in the killings last year of Chinese boat crews in the area.
The October 2011 murders were one of the deadliest assaults on Chinese nationals overseas in modern times and caused Beijing to send gunboat patrols to the region downstream from its borders.
After denying the charge in court and blaming the killings on Thai security forces, Kham reportedly changed his plea after the five other defendants testified he was the gang's leader.
The six suspects all faced charges of intentional homicide, drugs trafficking, kidnapping and hijacking.
According to China's official state broadcaster, Kham, in his 40s, pleaded guilty at the Intermediate People's Court of Kunming in south-west China during a two-day trial in which he "expressed his penitence to the victims and their families".
It added: "Prosecutors showed irrefutable and sufficient evidence of the criminal offences, including the six defendants' confessions, witness testimonies, DNA test results, and autopsy reports."
Kham, it said, was "hoping for leniency" and that the other defendants were "begging for lighter punishment" in pleading guilty.
The court has yet to announce a date for the sentencing.
The 13 Chinese sailors aboard two cargo ships were killed after being hijacked on a lawless stretch of the Mekong River last October. However, it has been far from an open-and-shut case to outside observers.
The prosecution claimed Kham was sending a warning to the hundreds of other ships that ply the Mekong, both legally and illegally, when he killed the sailors, as the two boats were suspected of trafficking narcotics not belonging to Kham.
However, a Reuters investigation in January showed that some, if not all, of the sailors were alive when their boats crossed into Thailand, and that they were executed and thrown overboard inside Thai territory – suggesting it occurred outwith Kham's sphere of influence.
In October last year, Thai police said that after speaking to more than 100 witnesses they were naming as suspects in the killings nine soldiers from a Thai army unit responsible for security along the Mekong.
Some witnesses described seeing the soldiers open fire on the boats. The Thai military responded by saying its troops had found the two boats, already riddled with bullets, on the river bank, with the captain of one slumped dead over his gun.
They reported finding 920,000 methamphetamine pills on board.
The nine Thai soldiers are still viewed by the police as suspects, although none has been charged.
Some believe Kham was a convenient scapegoat for the Chinese government as convicting him for a crime which outraged public opinion in China would be a popular move.
All Chinese shipping on the Mekong was suspended after the murders, with greater security co-operation among the four countries.
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