Purpose <p>Postoperative peritoneal adhesion formation (PAF) is a distressing complication. Leptin, secreted by fat tissues, may be associated with PAF. However, the relationships between PAF and leptin are unclear. This study investigated the impact of leptin on PAF after colorectal tumor resection.</p> Methods <p>A cecum cauterization mouse model was used to investigate PAF and its association with leptin. Mice adhesion score (AS) was evaluated on day 7 after treatment. The clinical study included patients who underwent elective colorectal resection with temporary ileostomy between April 2021 and December 2024. The AS was evaluated at the time of ileostomy closure. Patients with the highest AS quartile were classified into the high-AS group; clinical factors associated with PAF were examined. An enzyme-linked immunosorbent assay was used to measure human and murine serum leptin concentrations.</p> Results <p>The animal experiment revealed that serum leptin levels on postoperative day 7 were negatively correlated with mice ASs (<i>P</i> = 0.0043, <i>r</i> = -0.6241). In a clinical study, eight patients had a high AS (27.6%). Visceral fat mass and preoperative serum leptin levels were higher in the high-AS group than in the low-AS group. The serum leptin ratio (postoperative value to preoperative value) on postoperative day 7 was lower in the high-AS group. In multivariate analysis, the serum leptin ratio was an independent factor for classifying into the high-AS group.</p> Conclusions <p>A decrease in postoperative serum leptin levels was associated with severe PAF. Leptin may be a novel key molecule in PAF.</p>