Left-back Emanuelson moves to the Stadio Olimpico after his contract with Milan came to an end.
The deal contains an option for Roma to extend Emanuelson's stay for a further two years should he impress coach Rudi Garcia.
Emanuelson made 33 appearances for Milan last term, but the 18-time Serie A champions opted not to renew the 28-year-old's contract following a disappointing campaign in which they finished eighth in the Italian top flight.
However, Emanuelson's performances have convinced Roma to bring him to the Italian capital as Garcia aims to build a squad capable of competing on three fronts after securing UEFA Champions League qualification.
Emanuelson's capture comes four days after the arrival of former Chelsea full-back Ashley Cole, ensuring competition on the left-hand side of Roma's defence.
