OBJECTIVES: This study aims to evaluate the effect of obesity on radiological fracture union in diaphyseal femoral fractures (DFFs) treated with intramedullary nailing (IMN). PATIENTS AND METHODS: Between January 2017 and December 2018, a total of 120 patients (101 males, 19 females; mean age: 35.1±3.0 years; range, 18 to 72 years) treated with IMN for closed DFFs were retrospectively analyzed. Data including age, sex, location, weight, height, comorbidities such as diabetes mellitus, hypertension or kidney injury, date of injury, mechanism of injury, type of femoral fractures (AO classification), date of surgery, duration of surgery, IMN length and diameter used, date of radiological fracture union and complications of surgery such as nonunion, delayed union, and infections were recorded. RESULTS: Of the patients, 63 had obesity and 57 did not have obesity. There was a statistically significant difference in fracture configuration among patients with obesity; they sustained type B (p=0.001) and type C (p=0.024), the most severe fracture configuration. The nonunion rate was 45%. Obesity had a significant relationship with fracture nonunion with patients with obesity having the highest number of nonunion rates (n=40, 74.1%) compared to those without obesity (n=14, 25.9%) (p=0.001). Fracture union was observed within the first 180 days in 78.9% of patients without obesity, while it developed in the same time interval in only 38.1% of patients with obesity (p=0.001). CONCLUSION: Fracture union time for the patients with obesity was longer, regardless of the fracture configuration. Obesity strongly affects fracture union time in DFFs treated with an IMN. Obesity should be considered a relative risk in decision-making in the choice of fixation while treating midshaft femoral fractures.