The aid groups BEFEN and Health Alert say the workers were freed on Saturday near Niger's border with Mali. They say a sixth worker who had been kidnapped with the group was shot during the abduction and later died from his wounds.
Officials in Niger say the kidnappers are Islamists based in Mali, who have been linked to a string of abductions.
Mali plunged into chaos in March after a military coup toppled President Amadou Toure and led to the formation of an interim government.
Militants who seized the country's northern region have been trying to enforce Sharia law.
In October, the U.N. Security Council passed a resolution that clears the way for deployment of forces from the West African bloc ECOWAS. The regional group has proposed sending about 3,000 troops into Mali. VoA.