On-farm nitrogen (N) losses in pasture-based dairy systems are an environmental issue. Grape marc (GM) is a byproduct that contains condensed tannins (CT) and low crude protein concentrations, constituents that alter N partitioning in ruminants. The objective of this experiment was to determine if GM could decrease the urine N concentration and the urine N output (g N/d) from cows grazing a typical perennial ryegrass/white clover pasture. A second objective of the current experiment was to determine if the CT in GM are a component causing any changes in fecal and urine N concentrations through the use of polyethylene glycol (PEG), a CT inhibitor. The experiment was arranged in a 4 × 2 factorial design, including 4 levels of GM supplementation (0, 2, 4, and 6 kg DM/cow/d) and 2 levels of PEG (0, and 350 g/cow/d) in addition to their daily pasture allowance. Forty Friesian x Jersey crossbred multiparous cows in late lactation were evenly divided into one of 8 treatment groups. Cows were fed increasing amounts of GM for 10 d and fed the full treatment amounts of GM for a further 6 d. The pasture DMI were estimated for each treatment group daily, and intakes of GM and PEG were determined for each cow daily. Urine, feces, plasma and milk samples were collected on d 0, 10, and 16 of the trial. Fecal N % and DM % were analyzed, as well as urine N %, and concentrations of urine ammonia, urine urea-N, plasma urea-N, and milk urea-N. Estimated urine N excretion (g N/d) was calculated. Results were analyzed using GenStat. The dietary inclusion of PEG decreased fecal N % (P < 0.001), while dietary supplementation with GM lowered urine N % (P = 0.005) and urine urea (P = 0.003) concentrations. Cows fed GM also had lower plasma urea-N (P = 0.024) and milk urea-N (P = 0.026) concentrations. There was a tendency toward lower ammonia (mmol/L; P = 0.09) and N excretion in urine (g N/d; P = 0.08) due to the supplementation of dietary GM. These results indicate that the dietary inclusion of grape marc is effective at increasing fecal N and reducing urine N concentrations and that the CT within GM aids in increasing fecal N loss.