<p>The European green crab (<i>Carcinus maenas</i>) poses serious threats to native biodiversity and local fisheries. In the SW Atlantic, it has been rapidly expanding northwards; however, a regional demographic assessment is still lacking, which is essential for understanding invasion dynamics, assessing ecological impacts, and informing management strategies. We surveyed 19 coastal sites (40°–49° S) to evaluate green crab abundance, size, sex ratio, and colour morphs. We also examined the relationship between four environmental variables and crab abundance and size. Recently colonised areas (San Matías Gulf) exhibited higher crab abundance compared to regions with long-term population establishment. Within this gulf, crabs were more abundant on soft substrates, while larger individuals occurred on hard bottoms. Size varied regionally, with the largest crabs found in San José Gulf. Sex ratios differed among sites, showing a nonlinear, hump-shaped pattern. Males were more abundant and larger than females, while green morphs were more common than red morphs. No relationship was found between environmental variables and demographic traits, suggesting this species thrives under diverse conditions. This study constitutes the first comprehensive assessment of green crab populations in the SW Atlantic, providing a baseline for long-term monitoring and identifying areas at risk of ecological or economic impact.</p>