The spectrum of species of Sarcocystis in birds and the role of the latter in the transmission of coccidia are still incomplete for many avian species including Tengmalm’s owl Aegolius funereus (Linnaeus, 1758). Therefore, this study is focused on the morphological and molecular description of a new Sarcocystis species found in the intestine of the Tengmalm’s owl and its possible role as definitive host. Ten fledgling owls from the Kauhava region of west-central Finland were parasitized by numerous sporocysts and oocysts in the intestinal mucosa scrapings (prevalence 100%). Sporulated oocysts and sporocysts measured 16.34−16.96 × 11.47−12.09 µm and 11.85−13.52 × 7.77−9.25 µm, respectively. Skeletal and heart muscles were negative for sarcocysts. Sarcocystis funereus sp. nov. is closely related to Sarcocystis strixi from the barred owl (Strix varia Barton, 1799) from the USA and Sarcocystis sp. isolate 5 from the European shrew (Sorex araneus Linnaeus, 1758) from the Czech Republic. This is the first and most comprehensive record on Sarcocystis from owls obtained in Finland, thus highlighting the importance of molecular data in the species identification.