Cassia ferruginea (Schrad.) DC - Fabaceae, also known as "chuva de ouro", is one of the several native species recommended for the recovery of degraded areas in São Paulo state. In this work, 1H NMR analyses obtained from extracts of C. ferruginea seedlings identified flavonoid (+) - ampelopsin as the predominant compound in leaves and stems. This compound presents a well-established economic value due to its pharmacological properties and it has been commercialized as phytomedicine, cosmetics and analytical standard. However, there are no reports in the literature of chemical and biological studies of this specie or its commercial use for the production of phytomolecules. Due to the lack of analytical methodologies available for the quantification of (+) - ampelopsin in the ethanolic extracts of leaves and stems of C. ferruginea, as well as to promote value for reforestation areas, the objective of this work was to develop and validate an analytical methodology to quantify this flavonoid by NMR. In the context, a quantitative high - resolution NMR method (600 MHz) was developed and validated using the internal standard approach to measure (+) - ampelopsin in ethanolic extracts of leaves and stems of C. ferruginea, according to the “Resolution RE No 899’’.¹ Validation parameters such as linearity, selectivity (specificity), precision (repeatability), and robustness were determined. The method proposed by NMR was considered validated, since all validation parameters presented outstanding results for the proposed objective. Thus, the application of the high-resolution NMR technique through the ¹H experiment for quantification using the standard internal approach presented promising and coherent results, indicating a high content of (+) - ampelopsin in leaves and stems of seedlings (44.7 mg/g extract and 60.0 mg/g extract, respectively). These amounts are superior to those found in published papers from other sources (0.88 mg/g in fruit extract of Hovenia dulcis - Thunberg² and 0.01 mg/g in root extract of Ampelopsis sinica³). Thus, we conclude that a single measurement of 1H NMR could provide quantitative and structural information, demonstrating that the methodology is a useful tool to contribute to an efficient, simple and rapid quality control of plant extracts, besides it showed effective in segmentation and economic quantification of interesting molecules in native plants. Finally, the results reinforce that C. ferruginea seedlings can be used as a new and renewable source of (+) - ampelopsin.