The aim of the current study was to evaluate the effect of the presence of active proanthocyanidins (PAC) from sainfoin, either fresh or dehydrated included in the concentrate, on in vitro ruminal fermentation parameters, by including polyethylene glycol to block PAC. Total gas and methane productions, ammonia (NH3-N), and volatile fatty acids (VFA) were analyzed in fresh and in dehydrated sainfoin included in concentrates at different rates (0%, 20% and 40%), and ruminal biohydrogenation (BH) only in the latter. Active PAC from fresh and dehydrated sainfoin reduced the production of gas and methane (P < 0.01), with no effect on total VFA (P > 0.05). The presence of active PAC reduced NH3-N content in fresh sainfoin (P < 0.01), whereas the lower PAC contents in concentrates including dehydrated sainfoin only elicited a tendency (P = 0.06). The presence of sainfoin PAC in the concentrate decreased the BH extent and promoted the trans-11 BH pathway (P < 0.05). The inclusion of dehydrated sainfoin in the concentrate decreased the branched-chain fatty acids (FA; P < 0.01) and increased most of the trans-monounsaturated FA, C18:3 n-3, and BH intermediates (P < 0.05). In conclusion, the use of sainfoin in the diet of small ruminants can be a useful strategy to reduce gas and methane productions. In addition, the inclusion of dehydrated sainfoin in the concentrate produced changes in the ruminal FA profile that could promote a healthier meat FA profile.