Pinheiro, whose visit follows fast of the heels of UN special envoy Ibrahim Gambari, flew into Rangoon from Bangkok and was scheduled to travel immediately to Naypyidaw, the military's new capital, UN sources said.
Pinheiro is in Burma to assess the human rights situation in the country in the aftermath of a brutal crackdown on protests headed by Buddhist monks in September that left 10 ten dead, according to government figures.
Burma activists and Western observers in Rangoon believe the death toll from the September incident was closer to 200. More than 3,000 people have been arrested since late September, and unknown numbers remain under detention.
The International Committee of the Red Cross has been denied access to political prisoners in Burma jails for months.
Pinheiro is no stranger to Burma, which he has visited on several occasions in the past as UN rapporteur on human rights issues. The last time he was allowed a visa to the country was in 2003.
This time he arrived amid growing optimism, perhaps unwarranted, about Burma's political stalemate.
Gambari, who departed Burma Thursday, had persuaded the junta to allow opposition leader Aung San Suu Kyi to meet with her National League for Democracy colleagues on Friday and hold talks with the government's "minister of relations" Aung Kyi.
After Friday's meetings Suu Kyi expressed some optimism about the junta's willingness to start a process towards national reconciliation. The 1991 Nobel peace laureate has been under house arrest since May, 2003, and has spent 12 of the past 18 years in detention.
There is still great skepticism that the military, which has ruled Burma for the past 45 years, has any real intention of sharing power with civilian politicians in the near future.
Although Suu Kyi's party won the 1990 election by a landslide, the military has denied it any power for the past 17 years.
Gambari has the tough job of persuading Burma's xenophobic generals to initiate a dialogue which would ultimately loosen their iron grip on power.