Background: Leishmaniasis is one of the major zoonoses caused by infection with Leishmania protozoa, and is classified into three main clinical syndromes: cutaneous leishmaniasis, visceral leishmaniasis, and mucocutaneous/mucosal leishmaniasis. In particular, cutaneous leishmaniasis is a worldwide health problem and causes a range of diseases from self-healing infections to chronic disfiguring diseases. However, there is no vaccine for leishmaniasis, and drug therapy is often ineffective. Objective: This study aimed to search for new natural compounds with anti-leishmanial activity from marine organisms. Materials and Methods: Screening with 1565 extract collections prepared from marine organisms led to a red marine sponge designated S08216 collected at Tsushima Is., Nagasaki pref., Japan. The extract of the sponge was separated by reversed phase HPLC to yield the active substance. The structure of a new anti-leishmanial compound, aurantoside L was established on the basis of the interpretation of MS and NMR data. Results: Aurantoside L was a new tetramic acid glycoside showing the anti-leishmanial activity of IC₅₀ 0.74 µM against promastigotes of Leishmania amazonensis expressing egfp (La/egfp). It showed cytotoxicity of IC₅₀ 2.4 µM and 1.1 µM against HeLa cells and P388 cells, respectively.  Conclusion: Aurantoside L, a new anti-leishmanial tetramic acid glycoside, was isolated from the red marine sponge (S08216) and elucidated its structure.