Mats Gunnar "Mördaren" Thelin (born March 30, 1961 in Stockholm, Sweden) is a retired Swedish professional ice hockey defenceman who played 163 games in the National Hockey League for the Boston Bruins.
He played in Sweden for AIK Hockey from 1980 to 1985. He represented his country in the 1982 and 1983 World Championships, the 1984 Olympics, and the 1984 Canada Cup. He was chosen 140th overall in the 1981 NHL Entry Draft by the Boston Bruins.
After playing in the Canada Cup Thelin remained in North America and joined the Bruins. He played three seasons for Boston, splitting the 1985-86 season with the Moncton Golden Flames. In his three NHL seasons he played in 163 games and scored 8 goals and 19 assists.
In 1987 Thelin went back to Sweden and played for AIK until his retirement in 1994.