UN officials say 24 people were killed when the aircraft crashed into a hillside in thick jungle on Tuesday.
The Russian helicopter, leased by the UN peacekeeping mission in the West African country, was on a flight from Freetown when it crashed around five minutes away from its destination, the eastern town of Yengema, the UN said.
All those on board were killed, including three Russian pilots from the company that owned the aircraft, a UN official said at the world body's headquarters in New York.
"Military personnel confirm that there were no survivors," spokeswoman Marie Okabe said.
There was no official word on the cause of the crash or the nationalities of the passengers. But a peacekeeping official said 14 Pakistanis, several other foreigners and three Sierra Leoneans were among the dead.
Rescue aircraft spotted the burning wreckage of the aircraft some two hours after all contact with it was lost, the UN said. The rescuers could not land at the scene because vegetation was so thick.
The helicopter was a Russian Mi-8 MTV-1 from Russia's UTair company, one of the world's biggest helicopter operators which has been used by the UN missions from East Timor to Iraq.
"Secretary-General Kofi Annan extends his deep condolences to the families and governments of those who have perished in this tragedy," the UN said in a statement.
"He once again pays tribute to the men and women who have lost their lives in the names of peace in this and other important peacekeeping operations."
The UN has some 11,000 peacekeepers in its Sierra Leone mission, deployed to help end a decade-long civil war that became notorious worldwide for its brutality. Fighters frequently cut off the limbs of civilians, including children.
It was at least the third Russian-made helicopter used by the UN to crash in Sierra Leone in less than three years.
In November 2001, an Mi-8 helicopter plunged into the sea near the capital Freetown killing all seven people aboard after mechanical problems.
In October 2001 a British army press officer was killed in eastern Sierra Leone when the UN's Mi-24 helicopter crashed shortly after takeoff.