Data about all 18 known scorpion species of Sri Lanka are summarized. All previously known species were newly collected and four new species discovered during a scorpiological expedition in 2015. Information is provided about their taxonomy, distribution, ecology, and reproductive biology, fully complemented with color photos of live and preserved specimens, as well as their habitat. Subgenus Isometrus (Reddyanus) Vachon, 1972 is elevated to genus level, Reddyanus stat. n., supported by new characters of setation on leg tarsomere II and hemi-spermatophore morphology. Charmus saradieli sp. n., Reddyanus ceylonensis sp. n., R. jayarathnei sp. n., and R. ranawanai sp. n. are described, compared with other species and fully illustrated. Heterometrus serratus (Pocock, 1900) is restored and differentiated from H. indus (Geer, 1778). Charmus minor Lourenço, 2002 is synonymized with Charmus laneus Karsch, 1879; Isometrus garyi Lourenço et Huber, 2002 is synonymized with Reddyanus loebli (Vachon, 1982) comb. n., and Heterometrus spinifer solitarius Couzijn, 1981 is synonymized with Heterometrus indus (Geer, 1778). A key and distribution maps for all 18 Sri Lankan scorpion species are presented. Hemispermatophores of Buthoscorpio sarasinorum (Karsch, 1892), Charmus laneus Karsch, 1879, Isometrus thwaitesi Pocock, 1897, Lychas srilankensis Lourenço, 1997, Reddyanus basilicus (Karsch, 1879) comb. n., R. ceylonensis sp. n., R. loebli comb. n. and Heterometrus gravimanus (Pocock, 1894) are illustrated and described for the first time. In additional to morphological analysis, we also describe the karyotype of Buthoscorpio sarasinorum (2n=14); Charmus laneus (2n=9); Isometrus thwaitesi (2n=8); Lychas srilankensis (2n=16); Reddyanus basilicus comb. n. (2n=15–16); R. ceylonensis sp. n. (2n=16); and R. loebli comb. n. (2n=17). The significance of cytogenetics and hemispermatophore morphology in buthid taxonomy are discussed.