In this study, two previously unreported lignans [4′-O-demetylsuchilactone-8′-ol (1), 7-O-(3-methyl-2,3-dihydroxy-butyl)isodaurinol] (2), four new alkaloids (haplomegalanthine (3), megalantin (4), megalanthamide (5), glycoisohaplopine (6) and thirty-five known compounds (7-41) were isolated from Haplophyllum megalanthum Bornm). (Rutaceae).Pinoresinol (23), epipinoresinol (24), indole-3-carbaldehyde (29), N-[2-(4-hydroxyphenyl)ethyl]-3-methylbut-2-enamide (34), 7,8-dimethoxyfuroquinoline-4-one (35), cis-khellactone (36), 4,4′-dihydroxy-3,3′,9-trimethoxy-9,9′-epoxylignan (37), ulopterol (39) and trans-decursidinol (40) were identified for the first time in the genus Haplophyllum.Also, 4′-O-demethylsuchilactone (16) and N-(4-hydroxyphenethyl)benzamide (32) were found for the first time within the family Rutaceae.Structures of the compounds were elucidated by the use of HRMS, UHPLC-MS and 1D/2D NMR analyses.Furthermore, cytotoxic activity of the crude extract and previously unreported compounds (1-3, 5, 6) on colon cancer cell line HT29 were tested by the MTT method.IC50 values were determined for these five compounds and the crude extract as 18.48 ± 0.94 μg/mL, 52.62 ± 7.27 μg/mL, 23.15 ± 6.70 μg/mL, 21.45 ± 2.48 μg/mL, 5.06 ± 0.19 μg/mL and 2.82 ± 0.012 μg/mL, resp.The present report is the first detailed phytochem. investigation of the title plant pointing out that it comprises a rich source of diverse chem. constituents including new compounds with promising cytotoxic activity.