Grape marc (GM) is a byproduct of the wine-making industry and is a rich source of polyphenols. This experiment evaluated the efficacy of feeding supplementary GM to late-lactation dairy cows fed pasture-based diets as a means to manipulate the milk fatty acid (FA) and protein content produced. The effects of condensed tannins (CT) within GM were determined through the feeding of polyethylene glycol (PEG), a CT inhibitor. Forty late-lactation Friesian x Jersey cows were used in a 4 × 2 factorial design, receiving either (1) pasture only (control), (2) pasture + 350 g PEG/cow/d, (3) pasture + 2 kg DM GM/cow/d, (4) pasture + 2 kg DM GM/cow/d + 350 g PEG/cow/d, (5) pasture + 4 kg DM GM/cow/d, (6) pasture + 4 kg DM GM/cow/d+ 350 g PEG/cow/d, (7) pasture + 6 kg DM GM/cow/d, or (8) pasture + 6 kg DM GM/cow/d + 350 g PEG/cow/d. Cows were offered increasing amounts of GM for 10 d and maintained on the full amount of supplement for a further 6 d. Milk yield was determined at each milking. Milk samples were collected at a.m. and p.m. milking on d 0 and 16 and analyzed for milk components (milk fat, protein, and lactose), protein profile (α-, κ- and β- casein, α-lactalbumin, and β-lactoglobulin variants A and B) and FA profile by ANOVA using GenStat. Total milk and component yields were not affected by GM or PEG (P > 0.05). Of the short-chain FA analyzed (n = 13; C4 to C15), 63% were negatively affected (P < 0.05) by GM inclusion and 15% increased with PEG intake. PEG positively affected 25% of long-chain FA (n = 24; C17 to C26), while 29% were negatively affected by GM and 8% were positively affected by GM. Inclusion of GM did not affect the content (mg/mL) of any milk protein analyzed. Cows that received dietary PEG had lower α- (P = 0.06) and β- (P = 0.02) casein content than those not receiving PEG. In conclusion, GM negatively affected the profile of some short-chain (de novo synthesized) and long-chain (diet-derived) FA, the effects of which could not be fully eradicated by dietary PEG inclusion (CT inhibition). Dietary PEG inclusion decreased the content of α- and β- casein, suggesting that diet CT can increase the output of these proteins in milk.