Crop production practices affect weed communities in agricultural systems. Field experiments were conducted to determine the effects of organic and inorganic fertilization on weed flora and seed yield in black mustard [Brassica nigra (L.) Koch] crops. The experiments, conducted at two sites (Athens and Agrinio, Greece) during 2019, were laid out in a randomized complete block design with three replicates and three fertilization treatments (untreated, compost fertilizer and inorganic fertilizer). The results of the present study indicated that the total weed density and biomass in the black mustard crops were significantly influenced by fertilization. The highest leaf area index (4.76-4.84 m2 m-2) and canopy interception of incident photosynthetically active radiation (PAR) (93.56-94.98%) were observed under inorganic fertilization at 90 days after sowing. At both experimental sites, the total weed density and biomass were significantly higher under inorganic fertilizer treatment at 45 DAS. The highest densities of the weed species Sinapis arvensis, Chenopodium album and Malva sylvestris were found in the plots treated with inorganic fertilizer. The seed yield was also affected by fertilization, with the highest values (1352-1456 kg ha-1) observed in the plots treated with inorganic fertilizer. In conclusion, the different fertilization practices affected both the weed density and weed biomass; in particular, inorganic fertilization led to the development of he highest weed density. Furthermore, the highest seed yields were recorded under inorganic fertilization in both the experimental areas.