Russia has fought against Chechen
fighters for almost a decade
About 13 other people were wounded, some of them seriously when the woman detonated a belt of explosives strapped to her body.
The bus was traveling from the city of Mozdok in Ossetia to a nearby military air base.
Chechen police sources said the woman, who was dressed in a white medical coat, tried to wave down the bus in the early hours of the morning as it was leaving Mozdok.
"Her aim was to get inside the air base. But after they did not open the doors to the bus, she blew herself up," Russia's Deputy Prosecutor General, Sergei Fridinsky told Interfax news agency.
Mozdok is the main headquarters for Russian forces who have been fighting Chechen seperatists for the past decade.
It was the third attack by women suicide bomber in as many weeks as Chechen fighters increasingly use this form of attack in their war against Moscow.
"We have been unable to identify the suicide bomber so far - there was little left of her," Fridinsky said.
The breakaway republic has been fighting for almost 10 years in an effort to separate from Moscow.
But a Moscow spokesman for moderate Chechen rebel leader Aslan Maskhadov denied the separatist command had ordered the attack.
Two suicide attacks in May were claimed by leading radical Chechen rebel warlord Shamil Basayev.
Russian President Vladimir Putin has proposed amnesty to Chechen fighters who surrender. Parliament still has to approve the measure in a final reading this week.
Chechen fighters have vowed not to lay down their arms.
Putin's plan has been widely criticised by rights activists and observers who say the amnesty will cover only a minority of Chechen fighters. They say it will provide immunity for federal military and police.