Rantanen had 16 power-play goals, 33 power-play points and was plus-13 last season, his fourth in the NHL. He has increased his goals, assists and points each of the past three seasons and was the second-leading scorer for the Avalanche last season behind center Nathan MacKinnon (99 points; 41 goals, 58 assists) last season.
"We're really excited to get it over with," Avalanche captain Gabriel Landeskog said. "Obviously we wish it could have been done a little bit sooner. But we're excited that we're hopefully going to have him for the first game (of the regular season). He's going to be paying for a lot of dinner, so hopefully his credit card is ready to go."
Selected by Colorado with the No. 10 pick in the 2015 NHL Draft, Rantanen has 209 points (80 goals, 129 assists) in 239 regular-season games and 18 points (six goals, 12 assists) in 18 playoff games.
"Signing Mikko to a long-term deal has been a priority, and we are excited to have him under contract for the next six years," Avalanche general manager Joe Sakic said. "Mikko has established himself as one of the premier young wingers in the NHL and is a big part of our core group. We look forward to having him back with the team as we get ready to start the season."
Rantanen was among the high-profile restricted free agent forwards to sign in time to start the season, including Mitchell Marner of the Toronto Maple Leafs, Patrik Laine and Kyle Connor of the Winnipeg Jets, Matthew Tkachuk of the Calgary Flames, Brayden Point of the Tampa Bay Lightning and Brock Boeser of the Vancouver Canucks.
NHL.com independent correspondent Josh Clark contributed to this report