A phytochemical investigation of the aerial parts of Euphorbia cactus Ehrenb. ex Boiss. revealed a new megastigmane, euphocactoside (5), along with eleven known metabolites. Euphocactoside (5) is the 3-O-glucoside derivative of a polyhydroxylated megastigmane showing unprecedented structural features. The structure of euphocactoside, including stereochemical details, was elucidated by extensive spectroscopic analysis based on 1D and 2D nuclear magnetic resonance (NMR) and high-resolution mass spectrometry (HR-ESIMS). The isolated compounds were evaluated for their cytotoxic activity against three different human cancer cell lines, namely, A549 (lung), LoVo (colon), and MCF-7 (breast), using MTT assay, and moderate to marginal activities were observed for compounds 1–3, 8 and 9 against all three cell lines.