The men, including one American, were set free in the evening in the delta region, said Magnus Abe, spokesman for Rivers state government. Abe said he did not have information on who kidnapped the men.
An unidentified group of armed men snatched the six 10 days ago from nightclub in the centre of Nigeria's oil hub of Port Harcourt. The kidnappers arrived shooting into the air and sent scores of people rushing for cover. Officials said no one was injured in the incident.
Along with the American, the hostage group included two Britons, one Polish man, an Irishman and a Frenchman. They work for a range of oil and construction companies.
"I appreciate everything everybody's done for us. We are having champagne," Royce Parfait, of Tennessee, said by phone.
Hostage-takings have become nearly commonplace in Nigeria - Africa's largest crude producer and a major US supplier - where tensions between multinational oil firms and poor local communities have also sparked attacks on oil installations.
The hostages are usually taken by groups seeking political concessions or payouts and are rarely harmed.
Sixteen kidnappings in the past two weeks led Olusegun Obasanjo, the president, to declare a clampdown last week. The government arrested 160 people in the two-day crackdown, though most of those have since been released.
The six men had been snatched
from a club in Port Harcourt
Abe said a Lebanese man taken in a different incident remained in captivity. He earlier said that all hostages kidnapped in recent weeks had been freed.
In another development, Nigeria's oil workers' unions are considering pulling all their members out of the Niger delta after a series of abductions.
Two oil unions have called an emergency meeting after the kidnappings of 17 workers, mostly foreigners, in the last month. Oil companies have imposed strict travel restrictions on their staff to try to keep them safe.
The delta region is the hub of
Nigeria's lucrative oil sector
Government attempts to rescue a Nigerian hostage in the delta ended in a shoot-out on Sunday.
Up to 10 members of the Movement for the Emancipation of the Niger Delta were killed and it is still unclear whether the hostage, an employee of Royal Dutch Shell, survived.
"We are afraid for the safety of our members and anyone working in the Niger Delta. We feel the government is not doing enough," said Peter Esele, president of the Petroleum and Natural Gas Senior Staff Association of Nigeria.
Esele said the decision-making councils of the association and its ally, the blue-collar National Union of Petroleum and Natural Gas Workers, would vote on August 30 on whether to call on their members to leave the region.