Barks are today viewed as a potential resource for biorefineries given their chemical richness and diversity. This paper describes for the first time the chemical composition of Eucalyptus sideroxylon bark and the antioxidant properties of its polar extractives. The bark is thick, deeply furrowed and dark colored. Large pockets of kino were observed formed by the breaking down of tissues of the outer phloem. The mean chemical composition of E. sideroxylon bark was: ash 1.3%; total extractives 55.7%, mainly corresponding to polar compounds that were soluble in ethanol and water, lignin 13.1% and suberin 1.9%. The polysaccharides composition showed predominance of glucose and xylose (80.0% and 11.0% of total neutral monosaccharides respectively). The ethanol–water bark extract had a high content of phenolics: total phenolics 440.7 mg gallic acid equivalent/g extract, flavonoids 204.4 mg catechin equivalent/g extract and tannins 395.0 mg catechin equivalent/g extract. The antioxidant activity corresponded to 648.8 mg Trolox/g of extract, and FRAP values to 5247 mM Fe²⁺/g of extract. E. sideroxylon bark can therefore be extracted with green solvents to yield polar extractives with a potential valorization based on their chemical functionalities and bioactivity.