The present study was carried out to investigate the allelopathic effect of leaf and stem leachates of Parthenium hysterophorus on seed germination, seedling growth, plant height, and number of nodes of Pisum sativum. Different concentrations of leachates (0%, 20%, 40%, 60%, and 80%) were applied under controlled conditions. The results indicated that the control showed 100% seed germination, while increasing concentrations of leachates caused inhibitory effects. Maximum inhibition in germination was observed at 60% concentration, whereas slight variation was noted at 80%. Radicle and plumule length decreased progressively with increasing concentration, with maximum reduction recorded at 80%. Similarly, the number of nodes and plant height also declined significantly with higher concentrations, showing the strongest inhibitory effect at 60% and 80%. The study clearly demonstrates a concentration-dependent allelopathic effect of Parthenium hysterophorus, which may be attributed to the presence of phytotoxic chemicals. These findings suggest that this weed can adversely affect crop growth and productivity, highlighting its ecological and agricultural significance.