Hoefl-Riesch trailed Poutiainen after the first run, but had the fastest time in the second for a total time of 1 minute, 55.58 seconds. American teenager Mikaela Shiffrin was third, while defending overall World Cup champion Lindsey Vonn skipped the race to focus on upcoming races in Aspen, Colo.
"It's amazing, I'm so happy," said Hoefl-Riesch, who also won a slalom in Levi in 2009 and was second here in 2010. "I'm already looking forward to next year."
Poutiainen led by 0.
"Great to be on the podium with a home race," Poutiainen said
Hoefl-Riesch competed despite limited training because of an ailing hip. She said she wasn't getting enough speed on the flat portions of the course.
"So I knew I had to take a lot of risks on the steep," she said. "I had to attack."
The 17-year-old Shiffrin competed in her first race in Levi—the northernmost venue on the World Cup circuit, some 80 miles north of the Arctic Circle.
"I'm speechless, really," said Shiffrin, who was also third last in a slalom in Lienz, Austria. "I couldn't really ask for more.
Slovenia's Tina Maze, who won the giant slalom in the season opener two weeks in Soelden, Austria, was fourth to keep the lead in the overall World Cup standings with 150 points. Hoefl-Riesch is second overall with 124 points and Poutiainen is third with 91.
Austrian slalom specialist Marlies Schild went out in the first run when she straddled a gate just before the finish.
"I had a little mistake before the combination and then I was just straight and yeah ... I have to look at it with my coaches," Schild said.
Schild, the reigning World Cup slalom champion, won the Levi races in 2006 and 2010 and was looking to become the first woman to win three times at the Finnish venue.
Last season, she won six of 10 slaloms.