Magnesium is a critical mineral involved in numerous physiological processes, including neuromuscular function, enzyme activity, cardiovascular health, bone health, and the regulation of calcium and potassium balance. Magnesium has been shown to significantly help with diagnosis and prognosis in other species, yet its clinical significance in rabbits remains underexplored. This retrospective study analyzed serum magnesium concentrations in 457 rabbits, both healthy and diseased, to determine a normal serum magnesium in healthy rabbits, as well as its potential as a diagnostic tool. The reference interval was determined following ASVCP guidelines. The remaining rabbits were then divided into low, high, or normal magnesium based on the RI. For statistical analysis, all non-healthy rabbits were categorized into groups according to the primary organ system responsible for the patient’s clinical signs: gastrointestinal, cardiopulmonary, renal, metabolic disease, shock, and trauma. Magnesium concentrations were compared between disease groups and healthy rabbits. Differences in magnesium concentrations based on the system affected were found. Magnesium should be considered as a part of the diagnostic evaluation of pet rabbits.