<p>This study aimed to evaluate the correlation between serum phosphorus level and low muscle mass in patients with type 2 diabetes mellitus (T2DM). In this cross-sectional study, 1078 T2DM inpatients were recruited. Low muscle mass was defined as a skeletal muscle index (SMI) &lt; 7.0&#xa0;kg/m² for men and &lt; 5.4&#xa0;kg/m² for women, which measured by dual-energy X-ray absorptiometry. Serum phosphorus (P) was measured using the phosphomolybdate method. Clinical information and other biochemical characteristics were measured and recorded. The prevalence of low muscle mass was 20.2% in men and 13.6% in women. A higher serum phosphorus level was associated with a significantly lower risk of low muscle mass in both genders after multivariable adjustment. A nonlinear, threshold-dependent positive correlation was observed between serum phosphorus and SMI, with significant associations below cohort-specific thresholds of 0.98&#xa0;mmol/L in men and 1.04&#xa0;mmol/L in women. Higher serum phosphorus level is associated with a lower prevalence of low muscle mass (i.e., positively correlated with SMI) in T2DM patients. And this exploratory finding requires validation in longitudinal studies.</p>