Horse purslane (Trianthema portulacastrum L) a member of family Aizoaceae is a common weed in a host of crops such as maize, soybean, sugar cane vegetables and cotton in Pakistan and elsewhere. In order to quantify the impact of Trianthema portulacastrum on maize, experiments were conducted at Agricultural Research Farm, NWFP Agricultural University Peshawar, Pakistan for two crop seasons, viz. 2006 and 2007, using open pollinated variety “Azam”. The experiments were laid out in a Randomized Complete Block design with split plot arrangements, having, three replications per treatments. Four maize plant spacing (15, 20, 25 and 30 cm) were kept in main plots, while weed densities (3, 6, 9, 12, 15 and 18 m -2 ) were allocated as sub-plots. Data were recorded on days to 50% tasseling, days to 50% silking, days to maturity, plant height (cm), numbers of ears plant -1 , rows ear -1 , kernels ear -1 , ear weight (g), 1000-kernel weight (g), maize leaf area index, biological yield (t ha -1 ), grain yield (t ha -1 ), harvest index (%) and weed (T. portulacastrum) biomass (t ha -1 ). All growth parameters except for the number of ears plant -1 and rows ear -1 were affected by planting season, plant spacing and weed density. Plant spacing of 15 cm produced higher grain yields of 2.85 and 2.66 t ha -1 compared with 2.30 and 2.08 t ha -1 in wider plant spacing of 30 cm for the planting seasons of 2006 and 2007, respectively. However, for both years, yield components like ear weight and kernels ear -1 of individual plants were reduced with parallel decrease in plant spacing. Similarly, weed-free control plots produced higher grain yields of 3.04 and 2.87 t ha -1 vis-a-vis the grain yields of 2.14 and 2.0 t ha -1 in plots having weed density of 18 plants m -2 in 2006 and 2007, respectively. Plant spacing of 15 cm also resulted in higher biological yields of 6.96 and 6.69 t ha -1 in comparison with 30 cm spacing producing 5.92 and 5.85t ha -1 in 2006 and 2007, respectively. In the same vein, control plots had biological yields of 7.28 t ha -1 and 7.09 t ha -1 as compared with 5.81 t ha -1 and 5.65 t ha -1 in plots having weed density of 18 plants m -2 in 2006 and 2007, respectively. Plant spacing of 15 cm resulted in lower fresh weed biomass of 0.91and 0.88 t ha -1 compared with plant spacing5 of 30 cm producing higher weed biomass of 1.33 and 1.21t ha -1 in the growing seasons of 2006 and 2007, respectively. Likewise, the lower weed density of 3 plants m -2 resulted in lower weed biomass (0.97 t ha -1 in 2006 and 0.93 t ha -1 in 2007) compared with higher weed density of 18 plants m -2 (1.55 t ha -1 in 2006 and 1.49 t ha -1 in 2007). Decreasing plant spacing and increasing weed density of T. portulacastrum delayed tasseling, silking and maturity of grains. The T. portulacastrum infestations in plots having 3, 6, 9, 12, 15 and 18 plants m -2 resulted in yield losses of 4.2, 11.1, 18.6, 20.4, 27.2 and 29.5 % in 2006. The parallel figures for 2007 were 9.3, 14.3, 18.3, 23.2, 25.1 and 30.2 % in 2007. The two years of research showed that narrow spacing enhanced the competitive ability of maize crop and suppressed weed growth which eventually resulted in higher yields. Trianthema portulacastrum was a strong competitor to maize, and its infestation may inflict substantial yield losses, although the competitive ability was dependent on plant spacing and weed density. Plant spacing alone was not effective in suppressing T. portulacastrum; therefore, other cultural practices should also be integrated with optimum spacing to reduce the yield losses in maize crop. Key words: Trianthema portulacastrum, weed competition, maize.