An experiment was conducted during two consecutive boro seasons of 2015–16 and 2016–17 at the research farm of Regional Rainfed Lowland Rice Research Station, ICAR-National Rice Research Institute, Gerua, Hajo, Assam, India, to study the weed competitiveness of rice cultivars under two establishment methods. The split plot design with three replications was used with two rice establishment methods i.e., wet-seeded rice (WSR) and transplanted (TPR) in main plots and 10 hybrids and high yielding varieties (HYVs) of rice in subplots. The weed density of grasses, sedges and broad-leaved weeds (BLWs) at all growth stages were significantly higher with WSR as compared to TPR. The maximum weed density was recorded at 30 days after seeding (DAS) in WSR and at 15 DAS in TPR and later there was a decline in weed density due to the shift in crop-weed competition balance in favour of rice. The maximum weed biomass was observed at 60 DAS. The rice established by transplanting recorded higher growth, yield attributes, grain and straw yield. Among, the rice cultivars, Naveen recorded the lowest weed density and biomass at 30 DAS while Tulasi and Mandya Vijaya recorded the lowest weed density and biomass at 45 and 60 DAS which indicated their competitiveness against weeds. Due to better competitiveness, Naveen produced more vigorous plants and yield attributes which resulted in significantly higher grain and straw yield followed by Tulasi and KRH 2. Thus, for higher rice productivity in the shallow lowlands of Assam, transplanting of rice may be suggested using rice cultivars Naveen, Tulasi and KRH2 that were more competitive in suppressing weeds.