<p>Biological invasions cause animal suffering, but few studies assess these welfare impacts, and hence understanding of them is limited. We present a framework which can be used to identify relative changes to the welfare of an individual animal caused by biological invasions. We use it to assess the welfare impacts of bird and ant invasions. These impacts are a global phenomenon affecting native and introduced animals. Nevertheless, some introduced ant species cause severe impacts wherever they occur, whereas introduced birds do not. Impacts are likely to be underreported, particularly those affecting introduced animals. Physical and behavioural evidence (e.g., injuries and repetitive preening by birds) is sufficient to identify many welfare impacts. Physiological evidence (e.g., changes in ‘stress’ hormones) is more scarce, and could provide useful additional information to help quantify impact severity. Published biodiversity impacts of biological invasions are an unmined resource that may be used to assess impacts.</p>