AbstractCigarette butts (CBs) are the most common type of beach litter worldwide and contain a complex mixture of chemicals. Given the recent interest in this emerging problem, it is important to assess the toxicity of CB leachates to a range of species from different regions, sensitivities, and ecological traits. We evaluated the waterborne toxicity of smoked CB to tropical invertebrates. Leachates were prepared in the laboratory and characterized for trace elements (Mn, Fe, Co, Ni, Cu, Zn, As, Cd, and Pb), ammonia nitrogen, and polycyclic aromatic hydrocarbons. Then a set of toxicity tests with marine invertebrates was performed as follows: the brine shrimp Artemia sp. (nontoxic); the amphipod Tiburonella viscana (median lethal concentration [LC50] of 0.038 CB/L); the tanaid Monokalliapseudes schubarti (LC50 of 0.126 CB/L); the copepods Tisbe biminiensis (median effect concentration [EC50] of 0.038 CB/L) and Nitokra sp. (EC50 of 0.009 CB/L); pluteus stage larvae of the sea urchin Echinometra lucunter (EC50 of 0.152 CB/L); the sand dollar Mellita quinquiesperforata (EC50 of 0.054 CB/L); and D‐stage larvae of the mussel Perna perna (EC50 of 0.005 CB/L). The predicted no‐effect concentration was estimated using species sensitivity distribution, producing a 5th percentile hazard concentration of 0.015 CB/L. This preliminary threshold allowed us to estimate the potential impact of a single CB to 67 L of seawater via leaching, contributing to the advancement of knowledge regarding the contamination, toxicity, and ecological risks of cigarette waste. Environ Toxicol Chem 2024;43:374–384. © 2023 SETAC