Alternanthera brasiliana is popularly known in Brazil as "penicillin" or "benzetacil" and is used in traditional medicine for the treatment of infections and for the healing of wounds. It is also used as an ornamental plant due to the characteristic purple coloration of its leaves when cultivated in shade. The objective of this study was to develop and validate an analytical methodology by Ultraviolet-Visible absorption spectroscopy to quantify total flavonoids in crude ethanolic extract of A. brasiliana. The parameters analyzed in validation were those indicated in resolution 166/2017 of ANVISA, as selectivity/specificity, linearity, accuracy, precision, robustness, limits of detection and quantification, and also by ICH Q2(R1) for analytical validation. The method developed was simple, fast, low cost, linear, selective, precise, accuracy and robust. All parameters analyzed were within the limits recommended by the Brazilian legislation. Thus, this methodology can be useful for quality control of the extract and vegetal derivatives of A. brasiliana.