Three new tricyclic alkaloid, siamalkaloids A-C (1-3), together with three known alkaloids (3-6) were isolated from the twigs of Cassia siamea. Their structures were determined by means of HRESIMS and extensive 1D and 2D NMR spectroscopic studies. Compounds 1-6 were tested for their anti-tobacco mosaic virus (anti-TMV) activity, and compound 3 exhibited high anti-TMV activity with inhibition rate of 34.5%. This rate is higher than that of positive control. In addition, the cytotoxicity of compounds 1-6 were also tested, and compounds 2-6 showed weak activity with IC50 values ranging from 2.8 to 9.4 μM for some tested human tumor cell lines. ©2015 Published by Elsevier B.V. on behalf of Phytochemical Society of Europe. .