Sainfoin (Onobrychis viciifolia) is a tanniniferous legume that has been used as non-bloating forage for horses and ruminants for centuries, but so far its protein-rich seeds have not been utilized as a feedstuff for monogastric animals. In Europe, the main protein sources in diets for organic pigs are the grain legumes faba beans (Vicia faba) and peas (Pisum sativum), alongside the expensive high-quality components soybean cake, skimmed milk powder and potato protein. Because of the scarce supply of organic soybean cake and the highly variable yield of peas, alternative and locally produced protein sources are of great interest. As part of the EU Core Organic II research project ICOPP (improved contribution of local feed to support 100% organic feed supply to pigs and poultry), sainfoin seeds have been tested as a protein source for organic weaned piglets. In a feeding trial, a control diet, one diet containing 10% sainfoin seeds with hulls and two diets with 10 and 16% dehulled seeds, respectively (as fed basis) were fed to 137 piglets (crosses of [Pietrain×Duroc]×[Landrace×Large White]) during the 4-week post-weaning phase. The protein-rich components of the control diet were peas and soybean cake, which were partly substituted for sainfoin seeds in the experimental diets. Sainfoin seeds were found to contain 279 g kg−1 crude protein (388 g kg−1 when dehulled, as fed basis) with an amino acid profile suitable for pig feeding. Neither feed intake and body weight gain nor feed conversion ratio differed between treatments. It is concluded that sainfoin seeds can be a valuable protein source in moderate percentages of 10–16% in organic diets for weaned piglets. Where sainfoin can be cultivated easily and the seeds are harvested, using them as feed for piglets can therefore be recommended.