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. On the final point before she left the court, Errani pulled up and let a shot from Williams pass by her without even attempting to get to it.
“I'm sorry. You were unbelievable all week,” Errani told fans during the victory ceremony, as she brushed back tears. “I tried to do my best and stayed on the court only for you.”
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.
“I'm not 100 percent but I'm just kind of going on adrenaline,” Williams said.
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 and while Errani had a few chances to get back into the first 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.
When former Italian player Lea Pericoli asked Errani during the trophy presentation if she was “disappointed” with her showing, the crowd responded with boos and whistles. Pericoli then acknowledged that Errani's appearance in the final was an accomplishment by itself.
Injury permitting, Errani was slated to play the doubles final later with partner Roberta Vinci against Kveta Peschke and Katarina Srebotnik.
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.”