Ruotsalainen first played as a youth within hometown club,
Oulun Kärpät. As a 14-year old he played at the under-16 level in 2011–12, edging out teammate and future NHL player
Sebastian Aho, to lead the team in scoring with 24 goals and 59 points. He shared fourth place in the C Junior Championship with Alex Olkinuora of
Tappara. In the following 2012–13 season, Ruotsalainen played the majority of the year at the under-18 level, returning to strengthen the under-16 team for the postseason where he won the C Youth Championship series in 2013 with 27 points in just 11 games. Ruotsalainen continued his ascension up the junior levels playing the majority of the 2013–14 season at the under-20 league, the
Nuorten SM-liiga. Showing early offensive potential, Ruotsalainen was selected 47th overall at the
2014 KHL Junior Draft by
Metallurg Novokuznetsk. He continued to play in the Nuorten SM-liiga in the 2014–15 season. At the conclusion of the year, he secured his first professional
Liiga contract, agreeing to an optional three-year contract with
Ässät on 17 June 2015. In the
2015–16 season, Ruotsalainen made his Liiga debut in the opening round on 11 September 2015, against rival club
Lukko. He went scoreless in nine minutes of ice time. He later registered his first professional goal in a 3–1 defeat by the
Espoo Blues on 24 November 2015. By season's end, he totalled 10 points in 51 regular season games. He split the year continuing to play at the under-20 level, registering 32 points in just 18 games, to lead the league in points-per-game with 1.78. In the
2016–17 season, Ruotsalainen played in a reduced role with Ässät, posting just one goal in 34 games, with a solitary appearance in the postseason. In order to continue his development, Ruotsalainen left Ässät to sign a one-year contract with fellow Liiga club,
Ilves on 7 April 2017. In the
2017–18 season, he improved his play, producing nine goals and 20 points in 60 Liiga games. During the campaign he was signed to a two-year deal before again extending his tenure with Ilves on another two-year contract in the early stages of the
2018–19 season to remain contracted through 2022. In his second season with Ilves, Ruotsalainen accelerated his development in the Liiga, leading the club in scoring with 21 goals and 42 points at the
centre position, finishing ninth in Liiga scoring. During the season he scored his first career hat-trick, notching the feat in a 6–5 victory over
HIFK on 23 January 2019. In the playoffs, Ruotsalainen continued to lead Ilves with seven points in as many games. Undrafted, Ruotsalainen gained the interest from the
National Hockey League (NHL), using an out-clause in his contract with Ilves to sign a three-year, entry-level contract with the
Buffalo Sabres on 8 March 2019. He made his NHL debut with the team on 9 April 2021, in a game against the
Washington Capitals. Ruotsalainen scored his first career NHL goal on 11 April in a 5–3 victory against the
Philadelphia Flyers. As a restricted free agent from the Sabres following the
2021–22 season, Ruotsalainen opted to return to Europe by agreeing to a one-year contract with Swiss champions,
EHC Kloten of the
National League (NL), on 22 July 2022. On 6 April 2023, HC Lugano communicates an agreement with the player for a two-years contract. ==International play==