President Issoufou gave the assurance on Wednesday during an interview with ABC News channel.
Jeffery Woodke was kidnapped by armed militants in October 2016 from his home in Abalak in the Tahoua region of Niger, some 350 kilometres from capital Niamey.
“From the information we have, he is alive and he is good health,” Issoufou said in the interview.
“We will continue in any case to do everything to bring about the conditions for his release.”
Before his kidnap, Woodke – who speaks the local language Tamasheq fluently as well as Fula and Arabic – had run the aid group JEMET since 1992, helping the local Tuareg community.
Niger has been dogged by a jihadist war for years, with groups such as the IS having presence in the country.
In 2017, four US soldiers and at least four Nigerien troops were killed in the area in an ambush believed to have been carried out by a regional affiliate of the Islamic State.