Errani decided to play on after suffering a leg injury at the end of the first set, but she was made to pay as the world number one cruised to a 6-3 6-0 triumph.
The victory in Rome saw Williams pick up the 60th title of her career.
"I'm also sorry for Sara today. She really played great all week," the American is quoted as saying by BBC Sport. "I'm like a fine wine, I guess - at least my tennis is. My game gets better with age."
Williams will begin the defence of her French Open title when the tournament gets underway in Paris on May 25.