The world number one defended her title on the Rome clay, disappointing the home fans cheering on Italy's first representative in the showpiece since 1985.
Errani has also progressed to the final in the doubles competition, and fatigue may have played a part as she was dominated by the American.
Williams raced into a 4-1 first set lead on her way, winning 55 per cent of points on Errani's first serves on her way to taking the opener, despite hitting as many unforced errors as winners (16).
And Errani - who had picked up a thigh injury in the first set - wilted in the decider, taking a paltry five points in the entire set as Williams stormed to her third title of 2014.
The victory gives Williams firm momentum ahead of the French Open, which begins in a week's time.
After surrendering her titles in Charleston and Madrid earlier in the campaign, Williams extended her exceptional recent record on clay as Roland Garros looms.
Since 2012, the 32-year-old has played 55 matches on the surface - winning an astonishing 53.
She has also won the French Open in the previous two years she has won in Rome - 2002 and last year.