Karl Martin Widmark (born 19 March 1961) is a Swedish children's writer and teacher. His famous books about LasseMaja junior mysteries, in English named The Whodunit Detective Agency, have been translated from Swedish into 34 languages. Eight of them have been translated into English and published by Grosset and Dunlap. Several of his books about the Whodunit Detective Agency have been adapted to film in Sweden, and they have also been made into board games, theater and video games.

Martin Widmark was born in Sturefors and grew up in Linköping, but has lived in Stockholm since the 1980s. He has taught middle school and also Swedish for immigrants. He writes children's books and teaching materials including works on child literacy.

Martin Widmark is involved in issues concerning reading comprehension of children and young people and was the initiator of the project A Reading Class (En läsande klass) which was launched in 2012, together with several publishers and the Junibacken Foundation. He has made more than a thousand visits to different schools and classrooms and was in August 2014 awarded the Swedish government's medal Illis Quorum for his books and work with children and young people’s reading.

In the year 2000, Widmark made his debut with the book "To catch a tiger" (Att fånga en tiger). Since then he has written over hundreds of books for children and young people and is today one of Sweden's most popular children's book writers with almost 10 million books sold. His books have been translated into almost 40 different languages.