Roma announced on Tuesday that Fonseca will step down from his role at the end of the season to make way for the arrival of Mourinho.
It marks a swift return to management for the Portuguese, who was sacked by Tottenham on April 19 after a disappointing 17 months in charge.
He left Spurs with a 51 per cent win ratio in all competitions - only with Leiria (45 per cent) at the start of his managerial career had he posted a worse return.
However, Mourinho won two league titles, the Coppa Italia twice and a Champions League during his only previous spell in Italian football, which was with Inter between 2008 and 2010.