Fishermen who took the man to North Sentinel island say tribespeople shot him with arrows and left his body on the beach.
Local media say he was a missionary. He has been identified as John Allen Chau.
Contact with indigenous Andaman tribes living in isolation from the world is illegal. Estimates say the Sentinelese number between only about 50 and 150.
Seven fishermen have been arrested for illegally ferrying the American to the island, police say.
Local media have reported that he wanted to meet the tribe to preach Christianity to them.
“Police said Chau had previously visited North Sentinel island about four or five times with the help of local fishermen,” journalist Subir Bhaumik, who has been covering the islands for years, told BBC Hindi.
“The number of people belonging to the Sentinelese tribe is so low, they don’t even understand how to use money. It’s in fact illegal to have any sort of contact with them.”
In 2017, the Indian government also said taking photographs or making videos of the aboriginal Andaman tribes would be punishable with imprisonment of up to three years.
Firing arrows at helicopters
I first heard of the Sentinelese in 2004 just after the devastating Indian Ocean tsunami.
I was at the daily press briefing by the authorities a few days later when we were told that the members of the isolated tribes had survived.
A navy helicopter on patrol had flown over the North Sentinel island of the archipelago where the Sentinelese live to check on them.
As they descended a bit to take a closer look, members of the tribe began firing arrows at them.
“So we knew that they were safe,” the pilot told us.
The Sentinelese are often described as the most dangerous tribe in the world. They were among the first people to successfully migrate out of Africa and scientists believe they came to the Andaman islands 60,000 years ago.
In 2006, campaign group Survival International said the tribe had killed two fishermen who had illegally tried to enter their island.
“He was attacked by arrows but he continued walking.
“The fishermen saw the tribals tying a rope around his neck and dragging his body. They were scared and fled,” the report added.
Chau’s body was spotted on 20 November. According to the Hindustan Times, his remains have yet to be recovered.
“It’s a difficult case for the police,” says Mr Bhaumik. “You can’t even arrest the Sentinelese.”
Global organisation like London-based Survival International have been campaigning to protect the indigenous tribes living in the Andamans.
The group’s international director, Stephen Corry, called the incident a “tragedy” that “should never have been allowed to happen”.
“The Sentinelese have shown again and again that they want to be left alone, and their wishes should be respected,” he said. “The British colonial occupation of the Andaman Islands decimated the tribes living there, wiping out thousands of tribespeople, and only a fraction of the original population now survive. So the Sentinelese fear of outsiders is very understandable.”
The two endangered aboriginal Andaman tribes – the Jarawa and the Sentinelese – are hunter-gatherers, and contact with the outside world would put them at risk of contracting disease.
The Sentinelese are particularly vulnerable: their complete isolation means they are likely to have no immunity to even common illnesses such as flu and measles.
“It’s not impossible that the Sentinelese have just been infected by deadly pathogens to which they have no immunity, with the potential to wipe out the entire tribe,” said Mr Corry.