Phytochemical investigation on the 70% ethanol extract of the leaves of Alstonia mairei resulted in the isolation of three new monoterpenoid indole alkaloids, alstomairines A-C (1-3), along with one known compound, alpneumine A (4). Structural elucidation of all the compounds was accomplished by spectral methods such as 1D and 2D NMR, IR, UV, and HRESIMS. The isolated compounds were tested in vitro for cytotoxic activities against four osteosarcoma cell lines. Consequently, alkaloids 2 and 3 exhibited cytotoxic activities for all tested tumor cell lines with IC50 values from 9.2 to 13.0 μM.