The interest in knowledge of the weed communities of arable land is growing worldwide. Italy is one of the countries in Europe that is the most biodiverse, geographically and environmentally heterogeneous, and rich in arable weed species. Thus, in this study, the geo-environmental factors influencing the floristic composition, the species richness, and the Shannon diversity of weed communities of Italian winter arable crops were investigated along a gradient across mainland Italy. Original data were collected in the spring of 2018 in 106 winter cereal and legume arable fields from one fixed area plot per field. Environmental and geographic data were retrieved for each plot, including latitude, longitude, elevation, soil texture, soil pH, continentality, temperature, and precipitation. The effect of crop type was also tested. Latitude was the main driver of floristic differentiation between the studied plant communities, followed by precipitation, temperature, continentality, elevation, and longitude. Soil features and crop type had no significant effects. Higher values of species richness and Shannon diversity were found in southern areas and at higher elevations. Significant explanatory variables accounted for 11.55% of the total variation in species composition of the surveyed communities. The results are discussed and compared with those of similar studies in other Eurasian countries.