The hostages, including an American, a Briton, a Bulgarian and a Honduran, were then taken to the capital Abuja.
Still wearing their work shorts and overalls, they then met President Olusegun Obasanjo.
The President has given the men an assurance that Nigerians will do everything humanly possible to prevent a recurrence.
"We've had hostage takings in the past -- they were acting as terrorists -- but this is the longest and most traumatic case for you, your employers and even for those of us in government," he said, at a brief ceremony.
The men were later handed into the care of the energy giant Shell, for which they work as subcontractors.
The release came as a relief to workers in Africa's largest oil industry, which is reeling from three weeks of violent attacks that have left 22 police and soldiers dead and cut exports by more than 8%.
Rebels issue new threat
The militants have stressed that the hostages were only freed for humanitarian reasons and warned their struggle is far from over.
The Niger Delta's 14 million ethnic Ijaws want control over the region's oil and gas resources.
"This release does not signify a ceasefire or softening of our position to destroy the oil export capability of the Nigerian government," said the group in a statement from an email account used by the hostage takers.
"We repeat our warning to expatriates in the oil industry as they may not be as fortunate as these four individuals. Leave our land while you can," it said.
The rebels have threatened attacks to cut oil exports by 30 per cent by the end of February.
On January 11, a heavily armed group riding speedboats boarded the Liberty Service -- an oil industry supply vessel working under contract for the energy giant Shell -- and captured four crewmen.
Three of the hostages were employees of Tidewater, a Louisiana-based oil services firm: the boat's 61-year-old US skipper, Patrick Landry, and engineers Harry Ebanks, 54, from Honduras and Milko Yordanor Nitchev, 56, from Bulgaria.
The kidnapped Briton was Nigel Watson-Clark, a former paratrooper employed by the British company Ecodrill as a security expert.
"All of the workers will undergo medical examinations before repatriation to their homes and families," said a statement from Tidewater.
The kidnappers had demanded that the Nigerian government release two prominent Ijaw leaders from jail and that Shell pay 1.5 billion dollars (1.2 billion euros) in compensation to villages polluted by oil spills.
President Obasanjo said no deal had been struck, and added: "Some people wanted to put undue pressure on government so that they can get away with criminality."
Nigeria is Africa's biggest oil exporter, producing 2.6 million barrels of crude per day.
The crisis in the delta combined with fears of renewed instability in the Middle East has pushed prices towards historic highs.
On the same day as the hostages were taken, militants blew up Shell's Trans-Ramos pipeline.
Four days later, they stormed the firm's Benisede oil flow station, killed 14 soldiers and two oilmen, and burned down buildings.
Shell slashed production by 221,000 barrels per day.
It later warned tankers loading at its Forcados export terminal to expect delays of up to two weeks.
On January 24, gunmen wearing camouflage fatigues and armed with AK-47 Kalashnikov assault rifles stormed an office and workshop complex run by the Italian oil firm ENI in the southern oil city of Port Harcourt.
The gang gunned down eight policemen and a company accountant before escaping on speedboats with a big cash haul.
In an almost identical attack two days ago, gunmen scared off police guarding the South Korean engineering company Daewoo's oil services centre outside Port Harcourt and stole US$307,000 (A$409,000) in cash.
It is not clear if the robberies are linked to the separatists' political struggle, but the attacks bore all the hallmarks of Ijaw militant operations.