The article is devoted to a poorly studied problem of an ethnic shape of Muslims of the island of Sakhalin on materials of the First General Population Census of the Russian Empire in 1897. The island was the place of penal servitude where criminals from all the Russian Empire served their penalties. It resulted in a motley ethno-confessional structure of its population significantly differing from the other regions of the Russian Far East. The number of Muslims was rather considerable and exceeded the number of brothers in faith in the other Far East regions. Based on the census data analysis their gender, age and social structure is reconstructed.