Grape marc (GM) is a byproduct from grape pressing and contains condensed tannins. The objective of this experiment was to determine whether feeding GM to lactating dairy cows alters the milk proteome through changes in nitrogen (N) partitioning. Ten lactating Holstein cows were blocked by days in milk (141 ± 37 d) in a complete randomized block design. Cows were fed a TMR top-dressed with either 1.5 kg DM/cow/d GM (GM group) or 2.0 kg DM/cow/d of a 50:50 beet pulp:soy hulls mix (control group) for 28 d. DMI was recorded daily, blood samples were collected after AM and PM milking once weekly for plasma, and 24-h urine and fecal samples were collected on d 28. Milk samples were collected at AM and PM milkings thrice weekly for component analysis including milk urea N (MUN). Milk samples collected during the last week of the trial were composited within cow according to milk yield and subsequently skimmed. One aliquot of each skim milk sample was analyzed for high-abundance proteins using HPLC. A second aliquot was depleted of casein and fractionated. The whey fraction was enriched by ProteoMiner treatment before labeling with isobaric tandem mass tags and LC-MS-based peptide identification and quantification. Product ion spectra were searched on Proteome Discoverer 1.4 SEQUEST and Mascot search engines. Plasma urea nitrogen (PUN) was measured using a commercial kit. Urine and fecal samples were analyzed for N content. All results were analyzed using PROC MIXED of SAS (9.3) including repeated measures for DMI, milk yield and MUN. DMI (P = 0.61), milk yield (P = 0.16) and N partitioning were not affected by treatment (MUN, P = 0.61; urine urea, P = 0.34; fecal N, P = 0.41; PUN, P = 0.62). There was no difference in the high-abundance protein profiles across treatments. Of the 82 low-abundance proteins identified (peptide counts > 2), 16 were affected by treatment. GM did not affect known bioactive proteins including lactotransferrin (P = 0.23), osteopontin (P = 0.12), and lactoperoxidase (P = 0.14). These results indicate that low-level dietary supplementation with GM does not affect N partitioning or known key bioactive milk proteins.