The Burnout Assessment Tool (BAT) is gaining increased attention as a sound and innovative instrument for the conceptualization and assessment of burnout. This paper aims to present the psychometric properties of the Brazilian and Portuguese versions of the BAT; both the original 23-item and the shortened 12-item versions. Validity evidence for the BAT is based on the internal structure (dimensionality, reliability, and measurement invariance) and as well as on the relations to other variables. A cross-sectional study was conducted with two non-probabilistic convenience samples from two countries (N = 3103) one from Brazil (N = 2217) and one from Portugal (N = 886). The BAT’s original structure was confirmed, and it achieved measurement invariance across countries. Using both classic test theory and item response theory as frameworks, the BAT presented good validity evidence based on the internal structure. Furthermore, the BAT showed good convergent evidence (i.e., work engagement, co-worker support, role clarity, work overload, and negative change). In conclusion, the psychometric properties of the BAT make this freely available instrument a promising way to measure and compare burnout levels of Portuguese and Brazilian workers (download full paper)