Determining the competitive ability of a certain weed is a valuable guide as to what, where, and when best weed management techniques must be implemented. Likewise, identifying the critical period of weed-crop competition will help crop growers as to when weeds must be controlled or maybe allowed to compete with crops without having significant yield loss and unacceptable economic returns. Pot experiments were conducted to determine the competitive ability, critical period of competition, and density level of Hydrolea zeylanica (L.) Vahl under transplanted-irrigated lowland rice conditions. Treatments for the duration of weed-free period involved keeping rice plants free from H. zeylanica within periods of 10, 20, 30, 40, and 50 DAT; while the duration of tolerable period of competition involved allowing of the weed to compete with rice from 0 DAT, then simultaneously uprooted at 10, 20, 30, 40, and 50 DAT. In the critical density level experiment, H. zeylanica was allowed to compete with rice plants from 0 DAT until harvest at a ratio of 1:1, 1:3, 1:5, 1:7, 1:10, and 1:20 (rice: Hydrolea). Unweeded (control) and weed-free (check) treatments until harvest were included for comparison. Results showed that H. zeylanica was a very competitive weed against transplanted-irrigated lowland rice. When allowed to grow and compete, the weed significantly reduced rice grain yield and its vital agronomic parts. The critical period of competition based on classical method suggested that H. zeylanica can be allowed to compete only within 5 to 40 DAT and 10 to 30 DAT for minimum yield reductions of 5% and 10%, respectively. The estimations of Gompertz and logistic models suggested that H. zeylanica should not be allowed to compete within 0 to 700 GDD to achieve 100% relative grain yield. Under unavoidable circumstances in the field, however, the models suggested that the weed can be allowed to compete with transplanted rice within 0 to 670 GDD and 5 to 580 GDD to obtain minimal yield reductions of 5 and 10%. Furthermore, the negative effects of the weed on grain yield and yield components intensified when it was allowed to compete in dense population. Yields of rice that competed with H. zeylanica at 1:1, 1:3, 1:5, 1:7, 1:10, and 1:20 rice:weed ratios were reduced by 18.99, 24.68, 23.80, 35.46, 51.72, and 55.90%, in the same order.