The present study carried out a phytochemical investigation of the methanol extract of the branches and leaves of Clausena lansium and afforded nine carbazole alkaloids (compounds 1–9) including two new carbazole alkaloids, claulansiums A and B (compounds 1 and 2). The new compounds were elucidated on the basis of extensive spectroscopic data (MS, NMR, IR, and UV) and the known compounds were identified by comparing spectroscopic data with those reported in literature. All the isolated compounds were tested for their cytotoxic activity against A549 and Hela cancer cell lines. Our results showed that compounds 2–6 exhibited varying degrees of cytotoxicity to cancer cells, with IC 50 values ranging from 8.67 to 98.89 µmol·L −1.