Mr Maada Bio is a former soldier who briefly ruled Sierra Leone in 1996.
He won just under 52% of the vote and took the presidential oath in the capital Freetown on Wednesday.
He beat ruling party candidate Samura Kamara, an economist by training. President Ernest Bai Koroma has stepped down after serving two five-year terms.
Mr Maada Bio, who is leading the main opposition Sierra Leone People's Party (SLPP), lost to Mr Koroma in the last election. But Mr Maada Bio had already ruled the country briefly.
The 53-year-old was part of a group of soldiers who overthrew the government in 1992. Four years later he staged another coup and ruled Sierra Leone between January and March 1996.
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