Aim To evaluate maternal and cord blood irisin levels in pregnant women with gestational diabetes mellitus (GDM) and in obese pregnant women without GDM. Methods The study included 109 patients, with 34 patients in the GDM group, 40 in the obese non-GDM group, and 35 in the control group. Maternal serum irisin levels at the time of delivery were measured by an enzyme-linked immunosorbent assay kit. The correlation of serum irisin levels with metabolic parameters and anthropometric measurements was analyzed. Results There were significant differences between the study groups in terms of cord arterial, cord venous, and maternal serum irisin levels (P < 0.001, P < 0.01, P < 0.001, respectively). Cord arterial, cord venous, and maternal serum irisin levels were higher in the obese group compared to the control (P < 0.01, P < 0.01, P < 0.01, respectively) and the GDM group (P < 0.001, P < 0.001, P < 0.001, respectively). Conclusion Elevation in irisin levels of women who have pregnancies complicated with obesity may be explained as part of the compensation mechanism against disturbed metabolic functions. Pregnant individuals with GDM have lower serum irisin levels in comparison to healthy pregnant women. In this regard, it is possible that the measurement of serum irisin levels may be utilized in the future for prediction, prevention, and treatment of GDM.