Species introduced through human-related activities beyond their native range, termed alien species, have various impacts worldwide. The IUCN Environmental Impact Classification for Alien Taxa (EICAT) is a global standard to assess negative impacts of alien species on native biodiversity. Alien species can also positively affect biodiversity (for instance, through food and habitat provisioning or dispersal facilitation) but there is currently no standardized and evidence-based system to classify positive impacts. We fill this gap by proposing EICAT+, which uses 5 semiquantitative scenarios to categorize the magnitude of positive impacts, and describes underlying mechanisms. EICAT+ can be applied to all alien taxa at different spatial and organizational scales. The application of EICAT+ expands our understanding of the consequences of biological invasions and can inform conservation decisions. Agencia Estatal de Investigación PCI2018-092939, PCI2018-092986 Swiss National Science Foundation 31003A_179491, 31BD30_184114 Austrian Fonds zur Förderung der Wissenschaftlichen Forschung 4011-B32 German Bundesministerium für Bildung und Forschung 16LC1803A, 01LC1807C French National Research Agency ANR-18-EBI4-0001-06 US National Science Foundation ICER-1852060 National Research Foundation of South Africa 89967 Australian Research Council DP200101680 UK Natural Environment Research Council NE/V007548/1 Hellenic Foundation for Research and Innovation (H.F.R.I.) HFRIFM17-1597