A phytochemical investigation on the 90% ethanol aqueous extract of the aerial part of Kopsia arborea led to the isolation of three new monoterpenoid indole alkaloids, kopsiarborines A–C (1–3). The new structures were elucidated by using spectroscopic data (NMR, IR, UV, and MS). The isolated alkaloids were tested in vitro for cytotoxic potentials against six human lung cancer cell lines (A549, ATCC, H446, H460, H292, and 95-D). As a result, alkaloids 1 and 2 exhibited significant cytotoxic activities against all the tested tumor cell lines with IC50 values less than 20 µM. © 2019 Informa UK Limited, trading as Taylor & Francis Group.