Salah, who left Basle to join the Blues in January last year, became the first Egyptian to turn out for the Stamford Bridge side but has found regular football hard to come by.
He scored twice in 19 appearances for the reigning champions but had more joy out on loan at Fiorentina in the second half of last season, scoring nine times in 26 games.
Salah will now return to Serie A and Roma have the option to make the switch permanent.
“Chelsea Football Club has agreed terms with AS Roma for the season-long loan of Mohamed Salah,” Chelsea said in a statement.
“The winger will spend the 2015-16 campaign on loan at the Stadio Olimpico and the agreement includes an option to purchase at any point during the season.”
Roma expressed their delight on their website, saying the 23-year-old is widely regarded as “the best Egyptian player in the world right now”.
Salah thinks his new side can wrestle the Scudetto out of Juventus’ grasp.
“I think everybody here has ambition and everybody wants to win the title,” he said on Roma’s website.
“That would be good for us and very good for the fans. Everybody will do their best to make the fans happy.”