Mathias Klasenius is a Swedish international referee.

He mainly serves as an assistant referee in Swedish domestic league including Allsvenskan. He has experience refereeing Champions League and Europa League, also as an assistant referee.

He has been listed as a FIFA referee since 2007. He was an assistant referee in Euro 2012 and World Cup 2014. On 1 March 2016, he was named as an assistant referee for UEFA Euro 2016.