Major General Jioji Konrote's inauguration took place in a ceremony at Government House, Suva, presided over by Chief Justice Anthony Gates.
"I will dedicate myself to the wellbeing of the Republic of Fiji and all Fijians, protect and promote their rights," he said.
Parliament elected Konrote as president last month and he resigned as employment minister so he could take up the role.
The 67-year-old has previously commanded Fiji's UN peacekeeping forces in Lebanon and served as Suva's high commissioner to Australia from 2001-06.
More From This Section
The president's role under a constitution adopted in 2013 is essentially a ceremonial head of state, with most power resting with Prime Minister Voreqe Bainimarama.
Bainimarama, who led the 2006 coup, won office in elections held in September 2014.
Fiji has witnessed four coups since 1987 and the military remains a powerful institution in the fledgling democracy.
Fiji's police chief resigned abruptly earlier this week, accusing the military of interfering with his job.