Hemp (Cannabis sativa L.) is mainly grown for its fibre and is considered a desirable crop for sustainable production systems. In a field trial carried out over two years in Northern Italy the root system of a hemp crop, cultivated at contrasting plant densities, was sampled and analysed with an image analysis software. Root length density (RLD) was highest in the first 10 cm of soil, almost 5 cm cm−3; it decreased progressively until the depth of 130 cm, a part from a peak at 90–100 cm in response to a perched water table. Roots were found to 130 cm of depth in one year and to 200 cm in the other. Root diameter was finer (190 μm) in the upper soil layer, it increased with depth until 100 cm, and remained constant at 300 μm thereafter. Following the same trend of RLD, root biomass was highest in the first soil layer; 50% of the root biomass was found in the first 20 cm or 50 cm when taproot biomass was considered or not. Total root biomass was 3.21 t ha−1 and 2.41 t ha−1 in the two years of trial, but the ratio between aboveground and below ground biomass was constant at 5.46. None of the root parameters were significantly affected by plant population, which seems to confirm the plastic behaviour that hemp shows for aboveground development. The high root biomass production measured in this study, especially in deeper soil layers, provides additional evidence of the positive role that hemp can play in sustainable cropping systems.