Romanov signed a three-year contract with the New York Islanders on Aug. 22, 2022, after being acquired in a trade with the Montreal Canadiens on July 7 for the No. 13 pick in the 2022 NHL Draft (forward Frank Nazar). He signed an eight-year contract June 30, 2025, in lieu of becoming a restricted free agent, after ranking fourth among Islanders defensemen with 20 points (four goals, 16 assists) in 64 games and third in average ice time per game (22:18) while leading his position in hits (147) and blocked shots (165).

In 2021-22 season, Romanov led the Canadiens with 144 blocked shots and 227 hits, which ranked sixth among NHL defensemen.

Romanov was selected by the Canadiens in the second round (No. 38) of the 2018 NHL Draft and agreed to a three-year, entry-level contract May 8, 2020. Over two seasons (2018-20), he played 86 games for CSKA Moscow in the Kontinental Hockey League and eight playoff games, helping CSKA win the 2019 Gagarin Cup. Romanov played for his native Russia at the 2018 IIHF Under-18 World Championship and the IIHF World Junior Championship in 2019 (third place) and 2020 (second place). He was named the tournament's best defenseman in 2019 after leading all skaters with seven assists and defensemen with eight points.

After playing 54 of 56 regular-season games in 2020-21, his first NHL season, Romanov played in four of 22 Stanley Cup Playoff games during Montreal's run to the 2021 Stanley Cup Final. He scored his first NHL postseason goal at 8:48 of the third period in Game 4 of the Final, a 3-2 win against the Tampa Bay Lightning. At 21, he became the youngest defenseman in Canadiens history to score a goal in the Final.