Sergachev was selected in the first round (No. 9) of the 2016 NHL Draft by the Montreal Canadiens after he had 57 points (17 goals, 40 assists) for Windsor of the Ontario Hockey League. That season, his first in North America after playing in Russia, he finished third among defenseman in scoring and won the Max Kaminsky Trophy as the top defenseman.

On July 1, 2016, Sergachev signed a three-year, entry-level contract with the Canadiens.

He made the Canadiens out of training camp in 2016 and appeared in three of their first four games, making his NHL debut in a 4-1 win against the Buffalo Sabres on Oct. 13. He was assigned to Windsor on Oct. 31 and had 43 points (10 goals, 33 assists) in 50 games to help them win the 2017 Memorial Cup.

On June 15, 2017, Sergachev was acquired by the Tampa Bay Lightning in a trade with the Canadiens for forward Jonathan Drouin.

He scored 40 points (nine goals, 31 assists), had a plus-11 rating and averaged 15:22 of ice time in 79 games for the Lightning in 2017-18, scoring his first two NHL goals in a 2-0 win against the Columbus Blue Jackets on Oct. 19, 2017. In the 2018 Stanley Cup Playoffs, he had five points (two goals, three assists) in 17 games and became the youngest Lightning player to score a playoff goal.

The Lightning expanded Sergachev's role during the next two seasons. He reached double figures in goals (10) for the first time in 2019-20 and averaged an NHL career-high 20:22 of ice time. Sergachev saw even more time during Tampa Bay's run to the 2020 Stanley Cup, playing all 25 games and averaging 22:37 while scoring 10 points (three goals, seven assists).

NOTES & TRANSACTIONS

• OHL First All-Star Team (2016)
• OHL Second All-Star Team (2017)
• Traded to Tampa Bay by Montreal with a conditional 2nd round pick in 2018 NHL Draft for Jonathan Drouin and a conditional 6th round pick in 2018 NHL Draft, June 15, 2017.