Background: Bipolar disorder (BD) is often misdiagnosed, leading to poor treatment outcomes. Thus, accurate assessment of BD is of great importance, including in BD adolescents. The aim of the study was to explore the psychometric properties of the 33-item Hypomania Checklist-External Assessment (HCL-33-EA) in depressed adolescents. Methods: The study was conducted between March and November 2020 in Beijing, China. Depressed adolescents aged between 13 and 17 years (N = 260) with BD (N = 147) or major depressive disorder (MDD) (N = 113) diagnosed according to the International Classification of Diseases, Tenth Revision (ICD-10) were recruited. Patients’ hypomanic symptoms were assessed by their carers using the HCL-33-EA. Results: The HCL-33-EA showed high internal consistency (Cronbach’s alpha = 0.82) with two factorial dimensions. The Receiver Operating Characteristic (ROC) curve analysis revealed an area under the ROC curve (AUC) value of 0.61 (95% confidence interval (CI): 0.54–0.67). The optimal cut-off score of 7 generated the best combination of sensitivity (0.81) and specificity (0.37) for discriminating between adolescents with BD and MDD. Conclusion: The HCL-33-EA, with a two-factor structure, seems to be a useful tool for screening for BD in depressed adolescents. However, the high sensitivity and low specificity of the HCL-33-EA at the optimal cut-off value of 7 indicate that the HCL-33-EA needs to be further refined for young patients.