This study reports the effects of seasonal variation on the total polyphenol and flavonoid content and the in vitro antioxidant activity of Secondatia floribunda A. DC. The extracts were prepared from the inner bark and heartwood of samples harvested in the 2015 to 2016. The total phenolic and flavonoid content was determined by specific qualitative tests. The in vitro antioxidant capacity was analyzed using the following tests: 1-1-diphenyl-2-picrylhydrazyl radical (DPPH), 2,2′-azinobis-3-ethylbenzenothiazoline-6-sulfonic acid (ABTS), ferric reducing antioxidant power (FRAP) and iron (Fe) chelating activity. The total polyphenol and total flavonoid content varied over the harvest period. The DPPH and ABTS tests revealed that in the dry season had the most potent in vitro antioxidant activity, although the extracts obtained during the rainy season presented the higher Fe chelating and Ferric reducing activities. In conclusion, the phenolic content and in vitro antioxidant activity are correlated, and both are influenced by seasonality.