On March 26, in a game against the Vegas Golden Knights, he appeared in his 400th NHL game. For the sixth consecutive season, Barrie led Colorado's defence and set career-highs with 14 goals, 43 assists and 57 points in 68 games. His 57 points were the most by an Avalanche defenceman since Ray Bourque scored 59 points in 2001. Helping the Avalanche qualify for the 2018 playoffs, Barrie recorded four assists in six games in Colorado's first-round defeat to the Nashville Predators. Following the 2018–19 season, Barrie began to be featured in trade rumours due to the emergence of young Avalanche defencemen Samuel Girard, Cale Makar and Bowen Byram, the latter of which was drafted at the 2019 NHL Entry Draft. Additionally, there was belief that the team would be unable to sign Barrie to a new contract upon the expiration of his existing contract at the end of the 2019–20 season. On July 1, 2019, Barrie was traded to the Toronto Maple Leafs (along with Alexander Kerfoot and a sixth-round pick in the 2020 NHL Entry Draft) in exchange for Nazem Kadri, Calle Rosén and a third-round draft pick in 2020. Colorado also retained 50% of Barrie's cap hit as part of the trade. Upon arriving in Toronto, Barrie and Kadri lived in each other's houses as they transitioned onto their new teams. Barrie made his international debut at the 2008 World U-17 Hockey Challenge with Team Canada Pacific. Barrie was first invited to partake in Canada's national junior team development camp in 2010. He was again included to take part in Canada's 2011 national junior team selection camp and was among the final names to make the team. He was among the team's top players during the tournament as Canada won the silver medal following a 5–3 collapse defeat against Russia in the tournament final. Barrie would win gold with team Canada in the 2015 IIHF World Championship, along with then Colorado Avalanche teammates Matt Duchene, Nathan MacKinnon and Ryan O'Reilly. Following Colorado's failure to qualify for the 2017 playoffs, Barrie was selected to the initial Canada roster for the 2017 IIHF World Championship held in Germany and France. He was among the tournament leaders with seven points in three games before he was ruled out for the remainder of the tournament after suffering a laceration to his leg while wrestling with a Canada teammate at the team hotel on May 11, 2017. Barrie's grandfather Len Sr. runs a hockey school in British Columbia, which he volunteers at during the offseason.