This being his second five-year term in office, President Issoufou, was elected in a run-off vote last month where he polled 92.38 percent to beat his challenger Hama Amadou who secured 7.82 percent.
However, the main opposition in Niger boycotted the run-off vote on March 20.
With his hand on the Koran, President Issoufou took the oath of office at a ceremony attended by thousands of people.
The colourful ceremony which took place at the Stade du Seyni Kountché was witnessed by nine African Heads of State and Vice Presidents.
The Heads of State include President Idriss Deby of Chad, President Macky Sal of Senegal, Ibrahim Boubacar Keita of Mali, President Teodoro Obiang Nguema of Equitorial Guinea and President Denis Sassou Nguesso of Congo-Brazzaville.
The rest are President Alpha Conde of Guinea, President Alassane Ouattara of Ivory Coast, President José Mário Vaz of Guinea Bissau and President Thomas Boni Yayi of Benin.
Vice President Yemi Osinbajo of Nigerian, Prime Minister Ulisses Correia e Silva of Cape Verde as well as other foreign dignitaries were also at ceremony.
President Issoufou in his swearing-in address said the people of Niger made their choice in a calm vote and renewing their confidence in him for a second term mandate.
He also used the opportunity to highlight the country’s anti-terrorism fight in the landlocked West African country.
He said Niger is threatened by jihadist groups in the Sahel region to the north and west, and is also the target of attacks by Nigeria’s Boko Haram Islamists to the southeast.
“Terrorism is an absolute evil, stressing that as long as terrorism is not eradicated in northern Mali and as long as Libya is not stabilised, we will not be able to sleep in peace” he added.