In the current work, the experimental design aimed to assess the allelopathic effect of Alhagi graecorum Boiss. and to reveal bioactive metabolites that could interfere plant allelopathic interaction. Seeds of tomato,Solanum lycopersicum L., were used as allelopathic partner of A. graecorum. Averages of 46% inhibition in germination, 26.5 and 24% reductions in lengths of radicle and plumule of tomato's seeds were observed, respectively. Lower concentration (1%) of the aqueous extract was stimulatory for elongation of radicle and plumule. Whilst, the concentrations 2 and 4% were inhibitory. One way ANOVA revealed overall significant differences (P= 000). Post-Hoc analyses assured significant differences (P< 000) between all crosses when compared to controls. GC-MS analysis of dichloromethane extract of A. graecorum resulted in identification of 12 bioactive compounds, separated within 7.2 to 41.3 min from injection. Two major peaks and other minor peaks were detected. Allelopathic effect of A. graecorum could be attributed to some of these compounds. In addition, other identified compounds were already known to have medicinal contributions in many diseases and disorders. Conclusively, our results confirmed the allelopathic potential of A. graecorum on tomato's seeds. Additional studies on the biological activities of the identified compound are needed.