Aims: Low mood and anhedonia are the core symptoms of major depressive disorder (MDD). However, there is no established visual analogue scale that measures pervasiveness of both symptoms. We aimed to validate the Maudsley 3-item Visual Analogue Scale (M3VAS) as a measure of core depressive symptoms and suicidality. Methods: This is a cross-sectional secondary analysis combining data from two randomised controlled trials covering a broad range of depression severity from euthymia to severe depression. We validated the M3VAS by testing: 1) latent construct domains using factor analysis; 2) internal consistency using Cronbach's alpha; and 3) convergent validity by correlating M3VAS scores against scores on the Quick Inventory of Depressive Symptomatology-16 item (QIDS-SR-16), which is validated for use in clinical trials. Results: Of 180 patients in the combined cohort, 177 (98.3%) provided complete data on the M3VAS and QIDS-SR-16. The mean (SD) age was 41.6 (13.0) years and 59.3% were female. Using factor analysis, one eigenvalue above 1 was produced (2.39) that explained 79.6% of the variance, indicating a one-factor model. Cronbach's alpha was 0.87, demonstrating good internal consistency. Total M3VAS scores correlated strongly (r = 0.72, p<0.001) with QIDS-SR-16 scores, indicating good convergent validity. Limitations: This was a cross-sectional study and was not validated against a clinician-rated assessment for depression. Conclusion: The M3VAS is a simple, valid instrument for the assessment of core depressive symptoms and suicidality across the depression spectrum. Future studies should test the longitudinal validity of the M3VAS in detecting changes in core depressive symptoms and suicidality over time.