Lupinus campestris seeds grown in Mexico have a similar composition to soybean (44% protein, 13% lipids). Use of lupin seed in human and animal diets is limited by its quinolizidine alkaloids (QAs) and other antinutritional factor contents. The effect of aqueous, acid, and alkaline thermal debittering treatments with L. campestris seeds flour was evaluated on QAs, oligosaccharides (RFOs), and phenolic compounds (PCs) contents, as well as protein quality. The alkaline treatment most effectively eliminated QAs. Protein content increased from 430 g/kg in untreated seeds to 543 in the aqueous treatment, 567 in the alkaline treatment, and 563 g/kg in the acid treatment. RFOs were eliminated in all treatments. The obtained sample with alkaline treatment had the best protein quality (2.04); this value was 17% lower than that of casein (2.45). The apparent digestibility was over 90% in all treatments, with the aqueous treatment exhibiting the best value (93%).