Ruiz Castillo beat Hoefl-Riesch by 0.20 seconds in a race in which the top 17 finishers were separated by less than 1 second.
"So far it's been my best season and I'm incredibly happy about what happened today," said Ruiz Castillo, 31, who made it to the podium for the first time in 13 years.
Ruiz Castillo completed the 2,355-meter (1.46 miles) course at Meribel in 1 minute, 42.56 seconds. Marie Marchand-Arvier took third, just 0.001 seconds behind Hoefl-Riesch.
Julia Mancuso was the top U.S. finisher with a 1:43.46, which was good for 13th place.