The 20-year-old announced on Sunday that he was due to have a medical at the Serie A outfit, with the finalised deal completed on Monday.
Ucan, who has one cap for the Turkey national team, made 25 Turkish Super Lig appearances for Fenerbahce in the past two seasons and joins on an initial season-long loan.
"AS Roma would like to announce that it has agreed a loan deal with Fenerbahce Futbol AS for the player Salih Ucan for a fee of €4.75million," a statement on Roma's official website read.
"The contract is valid for the 2014-15 season and includes an option to extend the loan for another year, at no additional cost, and sign the player permanently from the 2016-17 season for a fee of €11million.
"Salih Ucan has therefore signed a five-year contract, which expires on June 30, 2019."
Ucan joined on the same day as left-back Ashley Cole, who arrived as a free agent following his release from Chelsea.