Background: Bipolar disorder (BD) is often misdiagnosed as major depressive disorder (MDD) in older patients. This study examined the psychometric properties of the 33-item Hypomania Checklist (HCL-33) and its accuracy to differentiate BD from MDD among older adults. Method: A total of 215 depressed older patients were recruited; 107 were diagnosed with BD (71 with BD-type I and 36 with BD-type II) and 108 with MDD. Principal components analysis (PCA) was used to explore the factor structure of the HCL-33. Cronbach’s alpha was calculated to test the internal consistency. Intra-class correlation coefficient (ICC) was used to measure test-retest reliability. The receiver operating characteristic (ROC) analysis was used to generate the optimal cut-off value to differentiate between BD and MDD. Results: Two factors were identified in the PCA analysis accounting for 33.9% of the total variance. The Cronbach’s alpha value for the HCL-33 was 0.912, with 0.922 for factor I and 0.664 for factor II. The test-retest reliability was excellent (ICC: 0.891). The optimal cut-off of the HCL-33 total score for discriminating between MDD and BD was 14, with a sensitivity of 88.8% and specificity of 82.4%. Conclusion: The HCL-33 had satisfactory reliability and validity and could be used to distinguish BD from MDD in older adults.