In a field experiment, all weeds including Trianthema portulacastrum L. (T. portulacastrum) and 200 plants/m2 of T. portulacastrum resulted in significantly higher weed/T. portulacastrum dry weight, growth rate and relative growth rate than those in other treatments except all weeds excluding T. portulacastrum. These treatments caused significant reduction in soybean dry weight, crop growth rate, relative growth rate, leaf area index, net assimilation rate and uptake of N, P and K by soybean. All weeds excluding T. portulacastrum were comparable with them on the reduction of most of these parameters. Competition by all weeds including T. portulacastrum resulted in significantly lower values of pods/plant, seeds/pod, seeds/plant and seed weight/plant of soybean than other treatments except 200 T. portulacastrum/m2. All the densities i. e. 25, 50, 75, 100 and 200 T. portulacastrum/ m2 and all weeds including and excluding T. portulacastrum resulted in significantly lower seed yield than weed-free check, but the reductions were greater in all weeds including T. portulacastrum and 200 T. portulacastrum/m2. There was significant negative correlation (at P≤ 0.01) between the T. portulacastrum density and soybean seed yield.