The objective of this study was to evaluate the effects of the inclusion of País grape marc (GM) in high (HF) and low forage (LF) ruminant diets on rumen methane (CH4) production and fermentation parameters in an in vitro (batch) culture. Experimental treatments replaced forage for GM at increasing concentrations (0, 10%, 20% dry matter [DM]) in the incubation substrates. Gas production (GP) and CH4 production were measured at 6, 12 and 24 h of incubation and in vitro DM disappearance, volatile fatty acids (VFA) and ammonia nitrogen (NH3-N) were determined at 24 h. The inclusion of 10% and 20% of GM reduced NH3-N concentrations (p < 0.001) by 48% and 58% respectively, and GM20 inclusion reduced BCVFA concentrations (p = 0.049) by 21% for HF and by 11% LF. Inclusion of 20% GM reduced the percentage of CH4 in gas (p = 0.038), CH4 production (p = 0.004) and CH4 yield (p = 0.027), but leads to a reduction of in vitro dry matter disappearance (IVDMD) (p < 0.001), gas production (p < 0.001) and total VFA (p = 0.001). In conclusion, in batch culture conditions, partial substitution of fibre sources by País GM in high and low fibre diets has the potential to reduce NH3-N concentrations and CH4 production and yield with a reduction of IVDMD.HIGHLIGHTS País GM reduces up to 58% in vitro NH3-N rumen concentrations regardless of forage level in the diet. Partial substitution of fibre sources with País GM reduces methane production by up to 15%. Partial substitution of forage sources with País grape marc has the potential to improve livestock systems sustainability.