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.
"I'm really sorry for today but I'll be back next year and try to do better," Errani said during the victory ceremony, as she brushed back tears.
Williams' other Rome titles came in 2002 and last year -- and she went on to win the French Open on both occasions.
"I'm also sorry for Sara today," Williams told the crowd in Italian. "She really played great all week."
A left thigh problem prompted Williams to withdraw before her quarterfinal match at the Madrid Open last week but now she appears back on track for the French.
Organizers attempted to whip up patriotic fervor by having the Italian anthem sung before the players walked out onto the court, and fans continuously chanted "Sara, Sara" to try and encourage Errani in an atmosphere that more resembled a Davis Cup or Fed Cup setting.
But Williams jumped out to a 3-0 lead in the first set and while Errani had a few chances to get back into the set, Williams overpowered the Italian with her serve -- she had seven aces to Errani's none -- and overall attacking game.
Still, Errani could console herself by becoming the first Italian runner-up in the tournament since Raffaella Reggi took the 1985 title in Taranto.
The last Italian man to win was Adriano Panatta back in 1976.
It was the 60th title of Williams' career, and a large portion of those trophies have come in recent years for the 32-year-old American.
Martina Navratilova holds the record with 167 titles and Williams needs seven more to match Billie Jean King for sixth place on the all-time list.
"I'm like a fine wine, I guess -- at least my tennis is,'' Williams said. "My game gets better with age.''