Campomanesia adamantium is native of the Brazilian Cerrado, and its leaves are popularly used as anti-inflammatory, antidiarrheal, and antiseptic in the urinary tract. This study aimed to develop and validate an analytical method by high-performance liquid chromatography for the simultaneous quantification and evaluation of the seasonal variability of gallic acid, catechin, and epicatechin, during 12 months, in C. adamantium leaf extracts. Chromatographic separations were performed with a mobile phase gradient of acetonitrile and 0.05% (v/v) trifluoroacetic acid solution, flow rate of 1.0 mL min-1, detection at 210 nm, C18 column (150 × 4.6 mm, 5 μm) and column oven temperature of 35 °C. The analytical method developed was selective, linear, precise, accurate, robust, and without matrix effect. The parameters obtained in the present study meet the requirements established by national and international guidelines. The best time for leaves harvesting, with the highest levels reached, was in September (0.0626%, m/v) and August (0.044%, m/v) for gallic acid, in June (0.3953%, m/v) and July (0.3804%, m/v) for catechin, and from May (0.1622%, m/v) and June (0.1415%, m/v) for epicatechin, in the dry season. Therefore, this study contributes by providing parameters for quality control of the raw material C. adamantium.