Ruotsalainen started his career in Kärpät. Nicknamed "Reksa", he was recognized as a talented defenseman while playing for Kärpät junior teams. Kärpät was promoted to SM-liiga from 1. Divisioona for the 1977–78 season, and Ruotsalainen played 30 games and posted 23 points in his first SM-liiga season. During the 1979–80 season, Ruotsalainen and Kärpät played in the SM-liiga Playoffs, but were eliminated from the Finals by HIFK. Kärpät, however, won the bronze medal series, 2-0, against TPS. The 1980–81 season proved to be a breakthrough for Ruotsalainen. Kärpät made the finals against Tappara, coming from behind in the fifth and decisive game, winning the game 5-2. After his first Championship in SM-liiga, Ruotsalainen headed to National Hockey League. Ruotsalainen was signed by the New York Rangers of the NHL in 1981. During his debut season, he scored 56 points in 78 games. Initially paired on defense with Barry Beck, Ruotsalainen was eventually moved to forward. Coach Herb Brooks said Ruotsalainen was the best skater in NHL. Ruotsalainen's fluid skating helped him compensate for his lack of size. Ruotsalainen had his NHL-career high in points during the 1984–85 NHL season, leading the Rangers in scoring with 73 points in 80 games. Ruotsalainen played in the 1986 NHL All-Star game. Ruotsalainen's contract expired after the conclusion of the 1985–86 NHL season, and he accepted an offer from SC Bern of the Swiss Nationalliga A. Ruotsalainen played in 35 games and scored 53 points for SC Bern before returning to the NHL. Ruotsalainen was signed by the Edmonton Oilers for the remainder of the 1986–87 NHL season and was given a spot on Defense. The Oilers went on to win the Stanley Cup at the end of the season, and Ruotsalainen's name was added to the Cup for the first time. Ruotsalainen next signed with HV71 in Sweden, with whom Ruotsalainen scored 32 points in 40 games. After his single season in Sweden, Ruotsalainen was again contracted by SC Bern, playing a single season for them, scoring 46 points in 36 games. Ruotsalainen returned to the NHL after a two-season absence, starting the 1989–90 NHL season with the New Jersey Devils. He was traded to Edmonton after 31 games, leading to winning his second Stanley Cup, as the Oilers won the Cup for the fifth time, the first without Wayne Gretzky. Ruotsalainen returned to Europe and played for SC Bern, yet again, in a tenure that lasted for three seasons. Ruotsalainen scored 38 points during his first season back in Bern. Ruotsalainen returned to his "home team" when he was contracted by Kärpät in 1993, by which time had been relegated back to 1. Divisioona. Ruotsalainen played four games for Kärpät, scoring 6 points. Ruotsalainen was acquired by SM-liiga team Tappara for reminder of the 1993–94 season. Ruotsalainen played six regular season games with Tappara and scored 6 points, and 3 points in 9 playoff games, as Tappara was eliminated by TPS in the semifinals. After his season in Finland, Ruotsalainen accepted a contract offer from SC Bern for the 1994–95 season, and played 19 games and had 10 points. Ruotsalainen returned to Finland for the 1995–96 season and played for KalPa with 8 points in 16 games. He was contracted mid-season to Swiss team ZSC Lions, where he played 18 games with 15 points. After his season in Zürich, Ruotsalainen ended his career in Finland, playing for Kärpät in the Finnish 1. Divisioona for two more seasons, scoring 31 points in 44 games during his Final season. Ruotsalainen ended his career in 1998. His jersey #10 is currently retired by Kärpät.
