The 30-year-old American overpowered the diminutive Errani, a French Open finalist and top-ranked doubles player in the world, registering 38 winners, including nine aces.
Williams cut loose with a squeal of joy after ending the 64-minute match with a second-serve ace.
"It's really awesome. This is what I really wanted and dreamed of all year," said the fourth-seeded Williams. "I'm in the final again and I'm really excited about it."
Fourteen-time Grand Slam singles winner Williams will be gunning to cap off a brilliant summer run with a fourth US Open title following her London triumphs at Wimbledon and the Olympics, where she won both singles and doubles gold.
"I think it will probably be the best summer I've ever had," she said when asked how she would feel if she won Saturday's final.
Williams has been in dominant form at Flushing Meadows, advancing without the loss of a set, dropping just 19 games in her six matches.
Errani did not have the weapons to keep Williams from powering in winners, even when she had the serve. The slender, 5-foot-4 Italian did not register an ace in the tournament.
Williams, owner of the biggest serve in women's tennis along with powerhouse groundstrokes, raced to a 5-0 lead and looked like she might duplicate her fourth-round 6-0 6-0 'double bagel' win against Andrea Hlavackova of the Czech Republic.
The Italian, however, won cheers when she avoided a shutout in the opening set by winning the sixth game when Williams netted a backhand.
Eranni held serve in the third and seventh games of the second set but was outscored overall 60 points to 35.
"She played a good match," the 25-year-old Errani said.
"She's a great player. I think maybe she's the best. For me, I think she is incredible."
Williams, in turn, complimented her opponent.
"She's such a good hope for Italy. She's such a good fighter," the American said. "I can see her improve so much.
She played really well."
Williams said she would not be carrying any added pressure into the final.
"I don't have anything to lose," she said. "I'm going up against the most successful player of the year in Victoria Azarenka. On paper I think she's been more consistent. It will be a good challenge for me."
A good challenge for Azarenka as well, who has lost nine of 10 matches against Williams.
Australian Open champion Azarenka of Belarus advanced with a 3-6 6-2 6-4 victory over French Open winner Maria Sharapova.