Asma Jahangir made the comments during a fact-finding mission in Brazil on Tuesday, where human rights activists say summary executions by police are commonplace.
"I can say with great sadness that I find a number of cases where police have been alleged to have committed extrajudicial killings and there is overwhelming evidence that that could have happened," Jahangir, the UN Special Rapporteur on Executions, said.
"I can also say that in these cases I find strong indications that the police get away with murder, literally with murder."
On Saturday, a 24-year-old man was gunned down in the northeastern state of Paraiba just days after he met with Jahangir.
The victim was believed to have had information that could identify members of a death squad operating in the area.
The UN envoy is scheduled to meet government and police officials in six states and the Federal District of Brasilia, as well as with human rights groups and relatives of death squad victims.
"I find strong indications that the police get away with murder, literally with murder"
UN special rapporteur on Executions
Jahangir's visit follows the release of a damning human rights report that denounced widespread abuses and corruption by Brazilian police forces.
The report by the Global Justice Centre and the Centre for Afro-Brazilian Studies documents the execution of 335 people in 24 of the country's 27 states since 1997.
It said authorities failed to take any action to bring the perpetrators to justice in 202 of those cases.