Malpighia emarginata is a tropical fruit plant, found naturally in the Caribbean islands and South America that produces an edible fruit known as acerola or Barbados Cherry. Its polysaccharides were obtained by aqueous extraction, subjected to a freezing and thawing process and ultrafiltration. A homogeneous fraction (ACWS-01E) was analyzed by sugar composition, HPSEC, methylation and NMR spectroscopy analyses. The results showed an arabinan-rich pectic polysaccharide, with 6.1 × 104 g/mol and formed mainly by a high methyl esterified (DM = 86%) homogalacturonan and branched arabinan. This latter is anchored in type I rhamnogalacturonan regions. The main chain of arabinan consisted of (1 → 5)-linked α-Araf, branched only at O-3. The potential ACWS-01E intracellular antioxidant activity against H2O2-induced oxidative stress in murine fibroblast cell line (3T3) was determined by DCFH-DA assay. The treatment with ACWS-01E significantly reduced H2O2-induced cytotoxic effect and the levels of reactive oxygen species (ROS). These findings suggested that ACWS-01E protected and improved NIH 3T3 cell viability from H2O2-induced toxicity by decreasing intracellular levels of ROS.