The woman, Kapari Leniata, a 20-year-old mother, was stripped, tied up and doused in petrol by the boy’s relatives in Mount Hagen in the Western Highlands, said the National newspaper.
She was then thrown onto a fire in front of hundreds of people.
Police and firefighters were unable to intervene, the paper said.
The Post Courier newspaper said police and firefighters had been outnumbered by the crowd and chased away. Both newspapers published graphic photographs of the incident on their front pages.
The provincial police commander, Superintendent Kaiglo Ambane told the National that police were treating the case as murder and would arrest those responsible.
In parts of the Pacific nation, deaths and mysterious illnesses are sometimes blamed on suspected sorcerers. Several reports have emerged in recent years of accused people, usually women, being killed.
In 2009, after a string of such killings, the chairman of Papua, New Guinea’s Constitutional Review and Law Reform Commission said defendants were using accusations of witchcraft as an excuse to kill people, and called for tougher legislation to tackle the issue.
The local Christian bishop, David Piso, told the National that sorcery-related killings were a growing problem, and urged the government “to come up with a law to stop such practice.”
The U.S. embassy in the capital, Port Moresby, condemned the killing as a “brutal murder”, the AFP news agency reports, and evidence of “pervasive gender-based violence” in Papua, New Guinea.
“There is no possible justification for this sort of violence. We hope that appropriate resources are devoted to identifying, prosecuting, and punishing those responsible for Ms. Leniata’s murder.”