Macrocystis pyrifera is a brown seaweed widely distributed on the coast of Chile, with a high content of polyphenolic compounds with antioxidant properties. The effects of its inclusion in the diet on oxidative stress in dairy cows have not been evaluated. Therefore, this study aimed to evaluate the effect of two doses of inclusion of Macrocystis pyrifera on pro-oxidant and antioxidant indicators in grazing dairy cows. Thirty-six mid-lactating and clinically healthy Holstein-Friesian cows were randomly assigned to one of three diets: Control (75% perennial ryegrass and 25% commercial concentrate); Treatment 1 (control diet and 1.5% inclusion of seaweed); and Treatment 2 (control diet and 3% inclusion of seaweed). Blood samples were taken by coccygeal venipuncture on days 18, 45, and 61 to determine the plasma concentration of malondialdehyde (MDA), Trolox equivalent antioxidant capacity (TEAC), and oxidant stress index (OSI). No significant differences were observed between groups regarding MDA, TEAC, and OSI. However, an interaction was observed, where at 45 and 61 days, the OSI of the cows was lower in Treatment 2 compared to the Control. Including 1.5% and 3% of Macrocystis pyrifera in the diet of grass-fed cows did not affect the oxidative stress indicators during mid-lactation.