Ficus deltoidea Jack, which has strong antioxidant properties, is widely consumed in traditional medicine as a treatment for various diseases. The current study aimed to investigate the oxidative stress biomarkers, such as 2-thiobarbituric acid reactive substances (TBARS), aldehydic and ketonic derivatives of oxidatively modified proteins, and total antioxidant capacity, as well as antioxidant defenses (activity of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), ceruloplasmin (CP)) in the equine erythrocytes and plasma to evaluate the antioxidant activities of the aqueous extract derived from leaves of Ficus deltoidea. The leaves of F. deltoidea were collected at M.M. Gryshko National Botanic Garden (Kyiv, Ukraine) and the Botanic Garden of Ivan Franko National University in Lviv (Lviv, Ukraine). Freshly collected leaves were washed, weighed, crushed, and homogenized in 0.1M phosphate buffer (pH 7.4) (in the proportion of 1:19, w/w) at room temperature. The extracts were then filtered and used for analysis. A volume of 0.1 mL of the plant extract was added to 1.9 mL of clean equine erythrocytes or plasma (the final concentration of the extract was 5 mg/mL). For positive control, 0.1 mL of phosphate buffer (pH 7.4) was used. The treatment of equine plasma and erythrocytes by extract derived from leaves of F. deltoidea resulted in reduced carbonyl derivatives of the oxidatively modified protein. The levels of lipid peroxidation were non-significantly changed. The incubation of equine plasma with an extract derived from leaves of F. deltoidea resulted in an increase in the activity of antioxidant enzymes such as superoxide dismutase, catalase, and glutathione peroxidase with a simultaneous decrease of ceruloplasmin level. The level of total antioxidant capacity was non-significantly increased. The results obtained suggested the antioxidant activity of the extract derived from the leaves of F. deltoidea plants exhibiting a decrease in the biomarkers of oxidative stress and enhancement of antioxidant defenses in the equine erythrocytes and plasma after in vitro treatment with the extract. However, further detailed investigation, especially in vivo and in vitro antioxidant studies is needed to justify the use of extract derived from leaves of F. deltoidea as a natural source of antioxidants.