The crude alkaloidal extract of Ixora undulata Roxb. leaves recorded a cytotoxicity of IC50 = 125 µg/mL against EL4 and revealed a reduction with CC50 = 47 µg/mL in the viability of MT-4 cells, beside a 50% protection with EC50 > 47 µg/mL against HIV-1IIIB. Bioassay guided fractionation of the crude alkaloidal extract of I. undulata leaves led to the isolation of a new dimeric thiazepine alkaloid, together with the monomer and its glycoside. The isolated alkaloids were structurally elucidated based on extensive nuclear magnetic resonance spectroscopy, infrared, and HRESI/MS. The isolated alkaloids were evaluated for their anti-HIV-1 and cytotoxic activities, while the obtained results revealed promising activity with structure-based relationship, which is discussed briefly.