Organically produced pork occupies only a small niche in the European meat market, with one of the main reasons being the shortage of locally produced high-quality protein sources. In an effort to promote currently under-utilized protein sources, two feeding trials were conducted with grass pea seeds as feed for weaned piglets. The grass pea (Lathyrus sativus L.) is a hardy grain legume that produces protein-rich seeds, but, as other grain legumes, it contains several anti-nutritive compounds. Apart from trypsin inhibitors and tannins, it also contains the neurotoxin β-N-oxalyl-L-α,β-diaminopropionic acid (ODAP) which may cause nerve damage in farm animals as well as humans. The content of both trypsin inhibitors and ODAP can be greatly reduced by hydrothermal treatment; therefore both raw and hydrothermally treated grass pea seeds were used in the two feeding trials. Diets were fed to 152 and 144 piglets [crosses of (Pietrain × Duroc) × (Landrace × Large White)] respectively, during their rearing phase directly after weaning. In experiment 1, a control diet was compared with two diets containing 10 and 20% raw grass pea seeds, plus one diet with 20% treated grass pea seeds, respectively (as fed basis). In experiment 2, a control diet was compared with one diet containing 20% raw grass pea seeds and two diets containing 20 and 30% treated grass pea seeds, respectively (as fed basis). Grass pea seeds contained 265 and 271 g kg−1 crude protein (as fed basis) in the first and second experiment, respectively, and had an amino acid profile similar to peas (Pisum sativum), including relatively low contents of methionine and cysteine. Hydrothermally treated grass pea seeds were found to be a valuable protein source that was well accepted by piglets. At an inclusion rate of 20–30% in diets, feed intake and daily weight gain of piglets was very similar to that of the control treatment. However, including 20% raw grass pea seeds had a significant negative effect on feed intake in experiment 1 and on daily weight gain in experiment 2. Therefore, hydrothermal treatment of grass pea seeds is recommended prior to feeding to piglets.