Touadera, 58, was the surprise winner of February's run-off election, which was held after three years of inter-communal violence between Muslim and Christian militias that left thousands dead and displaced more than 400,000.
"I pledge to wholly respect the constitution ... And preserve peace," he said, also promising to "carry out my duties without any ethnic bias."
The ceremony was held at the main stadium in the capital Bangui, which has a capacity of 20,000, and was attended by leaders from nearby countries, including President Teodoro Obiang Nguema of Equatorial Guinea and Denis Sassou Ngeusso from the Republic of Congo.
The elections came after 93 percent of voters backed a constitutional referendum that cleared the way for the vote and also followed Pope Francis' groundbreaking trip in November, his first to a war zone, during which he made an impassioned plea for peace and reconciliation.