A government spokesman said the hostages, an American, Briton, Bulgarian and Honduran, were released on Monday.
The spokesman for Nigeria's southern state of Bayelsa said: "They have all been released. They are all alive and well."
The four men were flown to the Nigerian capital, Abuja.
The hostages were abducted from an offshore oilfield in the Niger Delta operated by Royal Dutch Shell on 11 January.
The militants had demanded more local control over the delta's oil wealth, compensation for oil pollution and the release of two Ijaw leaders.
The Ijaw are the biggest ethnic group in the delta.
The militants renewed a threat to destroy oil rigs and pipelines and halt the country's key crude exports.
"This release does not signify a ceasefire or softening of our position to destroy the oil export capability of the Nigerian government"
The militants said on Monday that their release of the hostages for "humanitarian reasons" did not mark an end to the struggle by Niger Delta's 14 million ethnic Ijaws for control over the region's oil and gas resources.
"The release of the hostages was done purely on humanitarian grounds and no request was made for money," the Movement for the Emancipation of the Niger Delta said.
"This release does not signify a ceasefire or softening of our position to destroy the oil export capability of the Nigerian government.
"We will shortly carry out significant attacks aimed at ensuring our February target of a 30 per cent reduction of Nigeria's export capacity.
"We intend to hold captive any expatriates unfortunate [enough] to be found on any oil installations attacked. They will be released under no condition."
A militant source familiar with the situation said that $770,000 was paid for the hostages' release.