The German was second behind Poutiainen after the first run but had the fastest time in the second for a total time of 1 minute, 55.58 seconds.
"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 0.30 seconds after the first to raise hopes of a home victory, but couldn't keep up in the second and finished 0.55 behind Hoefl-Riesch.
Hoefl-Riesch competed despite being limited in training lately because of an ailing hip, and 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."
American teenager Mikaela Shiffrin held on to third place for the second podium finish of her career, 0.74 seconds back. This was 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 the 17-year-old Shiffrin, who was also third in a slalom in Lienz, Austria, last year. "I couldn't really ask for more. ... This is going to give me a lot of confidence."
Slovenia's Tina Maze, who won the giant slalom in the season opener in Soelden, Austria, two weeks ago, was fourth to stay first in the overall World Cup standings with 150 points. Defending overall World Cup champion Lindsey Vonn of the United States skipped the race.
Hoefl-Riesch is second overall with 124 points and Poutiainen is third with 91.
Marie-Michele Gagnon of Lac-Etchemin, Que., was 10th, while Erin Mielzynski of Guelph, Ont., tied for 14th.
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.