"We're all happy that it got sorted out really quick," the defenseman told the Flames website. "We love the city. We love the team. We love everything about the situation. I wanted to be here long-term, which is why I signed for six years. ... I feel like this is just the start of something good."
The 23-year-old could have become a restricted free agent after this season.
"We had real good dialogue," Calgary general manager Brad Treliving said. "We wanted to commit to him. He wanted to commit to us. When you start with that base, you can usually find a way to make things work.
"At the end of the day, it gives him great security. I think it's a great deal for Rasmus, and I think it'll be a real good deal for the Flames. He's only going to continue to grow, to evolve. The sky's the limit for this guy."
Andersson has 12 points (three goals, nine assists) and is minus-11 in 45 games this season, his second full season with Calgary.
"I feel like the coaches and management trust me, whoever I'm playing with," he said. "That's a good sign. And I think I'm up to that trust. I really do feel comfortable playing beside whoever they use me with. My defensive game has been really good the last two years but I feel I can still get more involved offensively from time to time."
Andersson was a second-round pick (No. 53) in the 2015 NHL Draft and made his NHL debut April 8, 2017.
