Errani had overcome Jelena Jankovic in the last-four stage to become the first Italian woman to reach the final since 1985.
However, chances of a fairytale victory for the local favourite were dashed as top seed Williams cruised to a 6-3 6-0 victory in 72 minutes to win the competition for a third time.
"I really want to compliment Serena on the way she's played during the week and she's certainly one of the best in the world, if not the best in the world," a tearful Errani told reporters.
"It's crazy for me that I got to the final this year. I never expected to manage to play such good tennis.
"I really want to thank my team who have always stood by me. I'm really sorry about today but hopefully next year I'll do better."
The triumph was Williams's third WTA Tour title of the season and 60th overall.