White lupin (Lupinus albus L.) grains, locally known in Anatolia as termiye, is traditionally boiled and soaked under running water for 7–10 days to remove quinolizidine alkaloids. However, little is known about its nutritive value for ruminants after this processing. In this study, water-debittered L. albus from five smallholder production lots (L1–L5) was characterised for chemical composition, Cornell Net Carbohydrate and Protein System (CNCPS) protein fractions, in situ degradability, in vitro fermentation kinetics, and predicted methane production. In situ degradability was determined in two rumen-cannulated dry Holstein cows over 2–48 h. In vitro fermentation was assessed using a semi-automatic gas production system. Rumen pH, ammonia nitrogen (NH₃-N) and volatile fatty acids (VFA) were measured at 24 and 48 h, and methane production was predicted from VFA molar proportions. Crude protein (CP) content ranged from 35.8 to 40.2% of dry matter (DM) and neutral detergent fibre (NDF) content from 35.3 to 36.9% of DM. CNCPS fraction B accounted for 88.0–93.2% of CP and rumen-undegradable protein (RUP) at k = 0.02/h was 30.6–32.9% of CP. Twenty-four-hour gas production ranged from 58 to 68 ml/460 mg DM, end-point pH 7.07–7.22, NH₃ N 9.2–10.0 mM, total VFA 125–132 mmol/l, acetate 55.9–58.2 mol%, propionate 26.2–27.1 mol%, and the (acetate + butyrate)/propionate ratio 2.54–2.69. Predicted methane ranged from 17.7 to 18.5 ml/460 mg DM. Water-debittered L. albus had high CP content, a true-protein-dominated nitrogen profile, RUP comparable to soybean meal, and an acetate-rich fermentation profile. Composition and fermentation characteristics were largely consistent between lots, indicating that traditional aqueous debittering results in a relatively uniform protein feed. However, confirmation of its feeding value requires in vivo evaluation.