Mattias Siimar (born 3 February 1998) is an Estonian tennis player.

[1] Siimar has a career high ATP singles ranking of 1036, achieved on 25 October 2021.

Playing for Estonia in Davis Cup, Siimar has a win–loss record of 5–6.

His twin brother Kristofer Siimar is also a tennis player.

This biographical article relating to Estonian tennis is a stub.