Depression may be highly prevalent among medical students, lowering their functioning and quality of life. Using appropriate extant depression scales to screen for depression and determining factors associated with depression can be helpful in managing it. This study examines the validity and reliability of the Patient Health Questionnaire-9 (PHQ-9) for medical students and the relationship between their scores and sociodemographic variables.This study surveyed 174 medical students using demographic questionnaires, the PHQ-9, the Beck Depression Inventory (BDI), the Patient Heath Questionnaire-15 (PHQ-15), the Beck Anxiety Inventory (BAI), and the Perceived Stress Scale (PSS). It calculated the Cronbach's α for internal consistency and Pearson's correlation coefficients for test-retest reliability and convergent validity of the PHQ-9. In order to examine the relationship between depression and demographic variables, this study performed independent t tests, one-way analysis of variance, chi-square, and binary logistic regressions.The PHQ-9 was reliable (Cronbach's α = 0.837, test-retest reliability, r = 0.650) and valid (r = 0.509-0.807) when employed with medical students. Total scores on the PHQ-9 were significantly higher among low-perceived academic achievers than among high-perceived academic achievers (p < 0.01). Depression was more prevalent in poor-perceived academic achievers than in high-perceived academic achievers. Similarly, poor-perceived academic achievers were at greater risk of depression than were high-perceived academic achievers (odds ratio [95 % confidence interval] 3.686 [1.092-12.439], p < 0.05).The PHQ-9 has satisfactory reliability and validity in medical students in South Korea. Depression is related to poor-perceived academic achievement when measured with the PHQ-9. Early screening for depression with the PHQ-9 in medical students and providing prompt management to high scorers may not only be beneficial to students' mental health but also improve their academic performance.