The world number one looked on top form against the 10th- seeded Errani, who was cheered on by a passionate home crowd but suffered a thigh injury towards the end of the first set and never recovered.
All the action was in the first set, which lasted longer than the 46-minute hammering Williams dished out to the Italian in the semi-finals of the French Open last year, when Errani won only one game.
The 27-year-old Errani, who has lost to Serena Williams, in all seven of their meetings, had to leave the court after losing her serve in the eighth game of the first set to get attention for her injury.
She was clearly less mobile after that, and the American rattled off seven easy games in a row to finish the match in one hour and 11 minutes, leaving Errani tearful in defeat.
"I'm sorry for Sara because she didn't deserve it to end like this, but I'm very happy for the win," said Williams in Italian on court.
Sunday's triumph was William's 60th WTA title and she dropped only one set on her way to victory, in Saturday's powerful semi-final display against Ana Ivanovic.
"She's obviously improved since Paris last year, and I think I could have played a lot better as well," Williams added later.
"I was a little nervous out there in the beginning, wasn't quite hitting my shots the way I should have been. That's exciting for me to know that I have room to grow as well."
Of Williams's victories, 23 have come since since she returned from a life-threatening pulmonary embolism in 2011.
"I'm like a fine wine I guess, or at least my tennis game is. It gets better with age. Hopefully that's how it goes," she said.
The 32-year-old had doubted she would start the tournament after pulling out of the Madrid Masters with a left thigh injury.
"I came to Rome eating pasta every day and hanging out with my friends, and then I thought OK, I'll practise on Tuesday and here I am. It was unexpected.
"But I had fun, and I think when I'm having fun is when I'm at my most dangerous."
Errani, who had been bidding to become the first Italian woman since Raffaella Reggi in 1985 to win the title, told the crowd: "I'm sorry, you've been brilliant all week."
"It was amazing for me to get to the final and play my best tennis thanks to you and my team, who have always been by my side," she added.
"Hopefully I'll be back next year and go one better."
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- Serena Williams