Objective: To evaluate the clinical performance of Glass Carbomer (GC) (GCP Dental) in restoring Class II cavities in primary molars in comparison with resin-modified glass-ionomer cement (RMGIC) and composite resin (CR) restorations. Method and materials: Healthy children aged between 4 and 8 years with a proximal lesion in at least one primary molar were recruited from the Pediatric Dental Clinics. A sample of 162 molars was randomly assigned to one of the following restoration types: GC, RMGIC, and CR. The restorations were evaluated clinically using the Cvar and Ryge criteria at 6 and 12 months postoperatively. Results: At 12 months, the success rates of anatomical form and marginal adaptation were 67% and 54% for GC, 98% and 93% for RMGIC, and 98% and 98% for CR, respectively. The GC restorations were significantly less successful than RMGIC and CR restorations in terms of anatomical form (P .001) and marginal adaptation (P .001). Secondary caries formation was not observed in any of the restorations in the three restorative material groups. Conclusion: The 12-month clinical performance of the GC restorative material was not satisfactory in restoring Class II cavities in primary molars. RMGIC and composite resin restorations performed significantly better. The use of GC cannot be recommended for restoring Class II cavities in primary molars. Keywords: composite, Class II, glass carbomer, glass-ionomer cement, primary molars