A field experiment was carried out at Pandit Jawaharlal Nehru College of Agriculture and Research Institute, Karaikal, U.T. of Puducherry, India during rainy season (Kharif) of 2019 with ten treatments replicated thrice in a randomized block design to study the critical period of crop-weed competition in irrigated blackgram in deltaic coastal ecosystem. The weed spectrum comprised of Echinochloa colona (L), Dactyloctenium aegyptium (L.), Trianthema portulacastrum (L.), Cleome viscosa (L.), Eclipta prostrata (L.) and Cyperus rotundus (L.). The density and dry weight of weeds significantly increased when crop-weed competition was prolonged from 15 days after sowing (DAS) to the maturity of the crop. The highest seed yield (706.5 kg/ in coastaha) was obtained when blackgram was maintained weed free till harvest closely followed by weed free till 60 DAS (652.1 kg/ha) and weedy condition till 15 DAS (608.6 kg/ha). The critical period of crop-weed competition was found to be 17 to 50 DAS. Weedy condition upto 15, 30, 45, 60 DAS and throughout crop growth resulted in a yield loss of 9.66, 39.19, 59.13, 75.87 and 86.30%, respectively.