Due to the 2012–13 NHL lockout, he started the season with the ŠHK 37 Piešťany in Slovakia, where he played eight games and recorded five goals and five assists.

Prior to the 2018–19 season, Tatar was traded to the Montreal Canadiens, along with prospect Nick Suzuki and a second round draft pick in 2019, in exchange for their captain Max Pacioretty.

[citation needed] In the Canadiens' run to the 2021 Stanley Cup Finals, Tatar was frequently a healthy scratch, playing only five games.

[17] Tatar made his Avalanche debut on the opening night of the 2023–24 season, registering an assist in a 5–2 victory over the Los Angeles Kings on 11 October.

[18] Slated to play in a top-six role, Tatar struggled to contribute offensively, scoring his lone goal with the Avalanche in his 26th game during a 6–5 victory over the Calgary Flames on 11 December.

[20] Tatar made his debut with the Kraken in an elevated role on the top-line alongside Matty Beniers and Jared McCann and saw his shootout attempt foiled in a 3–2 defeat to the Los Angeles Kings on 16 December 2023.

[21] As a free agent for a second consecutive year, Tatar opted to return to the Devils by signing a one-year, $1.8 million contract with the team on 2 July 2024.

[22] Tatar represented Slovakia at the 2009 World Junior Ice Hockey Championships, where he was the team's leading-scorer, and finished tied for fourth in tournament scoring, recording seven goals and four assists in seven games.

[23] Tatar was then selected to play for Slovakia at the 2014 Winter Olympics in Sochi, Russia, where he finished the tournament as the squad's second-leading scorer, with one goal and one assist.

[24] Tatar also represented Slovakia later that year at the 2014 IIHF World Championship, where he was the team's second-leading scorer, recording four goals and four assists in seven games.