The spatial pattern of weeds within field is widely accepted to be aggregated, but few studies have assessed the spatial structure of weed communities at the landscape scale. Using a combination of aggregation measures and similarity indices, this study aimed at inferring the processes behind the spatial patterning of weed communities. We analysed spatial variation in the species composition of weed communities to understand how similarity in weed composition varies with field and spatial effects. Weeds were surveyed in a regular grid sampling a 110 ha landscape mosaic made up of 24 fields. We showed that weed composition similarity among samples exhibited an aggregated pattern at the mosaic scale, of up to 400 m. Similarity in weed composition varied mainly among fields, but also within field according to the distance from the field border. The decay in species similarity was influenced both by field and spatial effects. Spatial effect remains signifi-cant after controlling for field effect. These findings provide support for the hypothesis that niche relationships and dispersal processes are both involved in the spatial patterning of weed species composition at the mosaic scale.