Guðmundur Pétursson Thorsteinsson (5 September 1891, Bíldudalur – 27 July 1924, Søllerød), better known as Muggur, was an Icelandic painter, graphic artist, author and film actor.

[3] In addition to his paintings, he wrote and illustrated a children's book, The Story of Dimmalimm, about a young girl and an enchanted swan.

[1] Later, Jóhannes úr Kötlum turned it into a verse play and it was also staged as a ballet, with music by Atli Heimir Sveinsson.

He was also a talented amateur actor and had a major role in one of Iceland's first films, Sons of the Soil (Saga Borgarættarinnar), based on a novel by Gunnar Gunnarsson.

[3] After a lengthy stay at a spa in France, he returned to the family estate in Denmark and died of tuberculosis, aged only thirty-two.