Financial terms were not released.
The 27-year-old forward had an NHL career-high 37 points (14 goals, 23 assists) in 80 games with the San Jose Sharks last season and three points (one goal, two assists) in 12 Stanley Cup Playoff games.
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"Joonas is a dependable player who plays 200 feet and uses his speed well in transition," Avalanche general manager Joe Sakic said. "He is coming off his best NHL season and brings playoff experience to our lineup. We are excited to have him under contract for the next four years."
Selected by the Florida Panthers in the fourth round (No. 99) of the 2010 NHL Draft, Donskoi has 122 points (45 goals, 77 assists) in 283 NHL games, all with the Sharks, and 21 points (nine goals, 12 assists) in 50 NHL playoff games.
