Five Americans, three Britons, two Indians, a Filipino and a South African were freed, as well as a Nigerian seized with them.
They were freed in Bayelsa state in the oil-rich Niger Delta, on what militants said were "humanitarian grounds".
One hostage said they had been subjected to mock executions.
Since January 2006, more than 180 foreign workers have been seized.
At least another 20 foreign workers are currently being held hostage in the region.
The government has pledged to address chronic under-development in the Niger Delta, which is home to Nigeria's multi-billion dollar oil industry, producing 90% of the country's export earnings.
The militants say they want more of the oil revenues to be spent on local communities.
One faction of the Movement for the Emancipation of the Niger Delta freed the group, who had been taken captive in different attacks by gunmen last month.
Although humanitarian reasons were cited, the BBC Nigeria correspondent Alex Last says state governments and oil companies usually pay ransoms for hostages' freedom.
The faction that released the men is based in the western Niger Delta and is currently embroiled in a feud with the main faction based in the east.
The fact that so many hostages were released at one time appears to be an attempt to show that it has influence and power across the region and is also therefore a key player in any deal to address the crisis in the area, our correspondent says.
'Three days of hell'
Bayelsa Governor Timipre Sylva said there were now no more hostages in the state.
He picked up the group by helicopter and hosted them at a dinner.
One hostage, South African Duplooy Smit, told Reuters news agency that "the first three days were hell".
He had been taken hostage from an oil industry vessel on 25 May.
"There were a lot of mock executions," he said. "They were all high on local gin and marijuana, and carrying machine guns, so you never knew what would happen next."