This study was conducted to evaluate the allelopathic effect of essential oil (EO) extracted from five Foeniculum vulgare Mill. cultivars against two weed species, Portulaca oleracea L. and Amaranthus retroflexus L. Various EO concentrations (0, 100, 200, 400, 600, and 800 mgL−1) were used to determine their effects on germination and seedling growth of P. oleracea and A. retroflexus. Radicle length, plumule length, radicle wet weight, plumule wet weight, plumule dry weight, and germination percentage of the target plants were measured. GC-MS analysis revealed the presence of different compounds in studied F. vulgare cultivars. Cultivars containing α-pinene possessed strong inhibitory potential towards weeds growth. On the contrary, cultivars containing fenchone showed poor inhibitory potential on both weeds. Moreover, the results indicate that with increasing EO concentration of F. vulgare, radicle length, plumule length, radicle wet weight, plumule wet weight, plumule dry weight, and germination percentage of both weed species decreased. The highest inhibitory effects were observed at 600 and 800 mg L−1 concentrations. Boushehr F. vulgare cultivar showed the highest inhibitory effect, while Esfahan and Yazd cultivars were identified as least inhibitory against the two weeds. Both P. oleracea and A. retroflexus showed similar sensitivities to F. vulgare EO.