Aim: To cross-culturally validate the ABC taxonomy into Portuguese for Portugal and Brazil without questioning its original meaning. Methods: A systematic review was conducted to identify published taxonomy terms and definitions in Portuguese, and to identify experts in medication adherence. Initial mapping of terms and definitions retrieved was scrutinized by the research team to build an e-survey. The e-survey was first piloted and then sent to experts in both countries seeking consensus using a three-round Delphi technique. Consensus was defined as ≥ 85% for the first round and ≥75% for the second. Terms with agreement lower than 10% were dropped between rounds. In the final round, terms and definitions reaching agreement between 50-75% were classified as moderate agreement and above 75% as strong agreement. Results: A total of 778 studies were identified and 84 included, enabling the extraction of 154 terms and 32 definitions. In the first-round 164 experts participated, 115 in the second-round and 99 in the third round. Strong consensus was obtained in Portugal for six terms and five definitions and moderate consensus for one term and two definitions. In Brazil, strong consensus was obtained for five terms and definitions and moderate consensus for two terms and definitions. Conclusion: A unified ABC taxonomy in Portuguese was possible to develop and validate for use in Portugal and in Brazil. Its use will harmonise and standardise the terms and definitions used in clinical practice and research.