Allelopathy is described as the interference to plant growth resulting from chemical interactions among plants and other organisms mediated through the release of bioactive secondary metabolites. Since only a few studies have been reported about the role of seed allelopathy, an experiment was designed to evaluate the interactions among seeds of Portulaca oleracea L. and the crop species common bean (Phaseolus vulgaris L.), onion (Allium cepa L.), sugar beet (Beta vulgaris L.), broad bean (Vicia faba L.), and pea (Pisum sativum L.) on seed and seedling growth parameters. The results indicated that P. oleracea seeds had a negative effect on the germination of P. vulgaris and A. cepa. Conversely, germination of P. oleracea in the presence of P. vulgaris, A. cepa, and B. vulgaris seeds was strongly reduced with a higher inhibitory effect found for the seeds of A. cepa. The highest negative effect on root and shoot length was observed in P. vulgaris. Seedling vigor of all crop species decreased in the presence of P. oleracea. Our results suggest that seeds of P. vulgaris, A. cepa, and B. vulgaris exhibited high allelopathic effects against seeds of P. oleracea and can be used as potential bio-herbicides in future screening programs.