Pellegrini emerged from Roma's academy before leaving for Sassuolo in 2015, where he has since scored 11 goals in 54 appearances across all competitions for the Serie A side, earning a maiden Italy call-up this season.
Having made his Azzurri bow against Liechtenstein in June, Pellegrini also helped Italy's Under-21s to the semi-finals of the European Championships, where they were beaten by Spain.
Having penned a five-year deal with Roma, Pellegrini is determined to leave his mark on the Giallorossi in his second spell, where he will link up again with former Sassuolo coach Eusebio di Francesco.
"It is an incredible feeling to come back," Pellegrini said. "This was my objective from the moment I went to Sassuolo. It is the culmination of a great two-year journey with them.
"I want to thank Roma, including Monchi and the rest of the directors. They have always been in close contact with me. They made me realise how much they wanted me back."
Federico Ricci has departed Roma for Sassuolo permanently after spending last season on loan at Mapei Stadium, while the Giallorossi have also allowed Seydou Doumbia to join Sporting CP on a season-long loan with a view to a permanent switch.