After Demidov had 49 points for SKA St. Petersburg to break Kirill Kaprizov's record (42) for most in a season by an under-20 player in the Kontinental Hockey League, he signed his entry-level contract with the Canadiens on April 8, 2025. He had a goal and an assist in his NHL debut six days later, when the Canadiens lost 4-3 in a shootout to the Chicago Blackhawks at Bell Centre. His productive rookie season continued when he was named the NHL's "Rookie of the Month" for December after leading first-year players with 14 points (four goals, 10 assists) in 15 games.

Demidov was then named a finalist for the 2026 Calder Trophy given to the NHL rookie of the year after leading first-year players with 43 assists, 62 points and 20 power-play points.

He proved to be a tremendous skater and excellent puck-handler playing for SKA St. Petersburg Jr. in Russia's top junior league. He had 60 points (23 goals, 37 assists), six game-winning goals and 144 shots on goal in 30 games during the 2023-24 season, and the Canadiens enlisted pop star Celine Dion to announce him as their selection with the No. 5 pick in the 2024 NHL Draft.

Demidov also excelled in the MHL playoffs; he had 28 points in 17 games to help SKA become champions. The Sergiyev Posad, Russia, native was named Most Valuable Player of the playoffs for the second consecutive season. He also won gold at the 2021 Hlinka Gretzky Cup with five points (two goals, three assists) in five games.