Errani was bidding to become the first Italian to win the tournament in nearly 30 years but the top-ranked Williams quickly took control in both sets and Errani appeared slowed by a left thigh problem.
Errani left the court for an injury timeout while trailing 5-3 in the opening set and came back with her thigh bandaged.
Williams' other Rome titles came in 2002 and last year -- she went on to win the French Open on both occasions.
In the men's final, seven-time champion Rafael Nadal was facing Novak Djokovic in a 1 vs. 2 meeting.
Roland Garros starts next week.