<p>The conservation efforts for Indian mithun <i>(Bos frontalis)</i>, a vulnerable and rare ruminant species exclusively found in the northeastern hill region (NEHR) of India, require a robust hands-on tool to appraise energy reserves which remarkably influence immunity and can affect survival and reproductive outputs. Given the difficulty in restraining mithuns in their wild habitats and their extremely aggressive nature, body condition score (BCS) is a reliable objective tool to assess health status; however, to date, no BCS index is developed for Indian mithun. We designed and validated a visual 5-point index, the Mithun Body Condition Score System (MBCSS). This index was designed based on the soft tissue around seven skeletal checkpoints (pins, hooks, ribs, short ribs, body outline, tail-head area, twist area) of adult mithuns (<i>n</i> = 30), with scores ranging from 1 (very thin) to 5 (obese). To guide an evaluator through the scoring process, MBCSS contains representative images of checkpoints corresponding to different BCS. For validation, ultrasound backfat thickness (uBFT) and serum NEFA and glucose were measured as two standard metrics of body condition. The mean uBFT and BCS were significantly (R² = 0.97, <i>P</i> = 0.001) correlated, indicating that MBCSS adequately reflected the body reserves. The scatter plots of BCS and NEFA and glucose concentrations also exhibited strong correlations. To evaluate intra- and inter-observer reliability, three observers from different professional backgrounds scored mithuns, and results were compared. Overall agreement values (%) for repeat assessment for all the observers demonstrated high suitability of this index. The results illustrate that MBCSS is an accurate and easy-to-use management tool to monitor body condition, maintain dwindling mithun populations at a target BCS, and protect them from becoming endangered.</p>