Bartoli, 19, who had never been past round 16 in the Grand Slam event, dropped the first set 6-1 in 22 minutes but came back to knock out No. 1-seeded Justin Henin 1-6, 7-5, 6-1 in semifinal action.
Henin, who won the French Open last month but missed the 2007 Australia Open for personal reasons, had played in the finals of the last five Grand Slams events she entered.
Williams, seeded 23rd, has put together an amazing run as she's reached the finals at Wimbledon for the sixth time. Williams knocked out second-seeded Maria Sharapova, fifth-seeded Svetlana Kuznetsova and sixth-seeded Ana Ivanovic, all in straight sets.
She was on the court just 85 minutes in the semifinal 6-2, 6-4 victory over Ivanovic on Friday, committing just 11 unforced errors in the rout.
Williams, 27, in Saturday's finals will be looking for her fourth Wimbledon title. She won the event in 2000, 2001 and 2005 and was the runner-up in 2002 and 2003.
Bartoli and Williams have never before played against each other.