This short-term longitudinal study investigated the developmental trajectory of first-year students' depressive mood and the role of personality dimensions after the transition to high school. Chinese first-year high school students (251 males, 295 females) completed surveys to determine depressive mood and personality dimensions. The latent growth modeling results were as follows: participants' depressive mood increased linearly after the transition to high school, and the intercept and slope of depressive mood were insignificantly correlated; neuroticism significantly and positively predicted the intercept and slope of depressive mood; extraversion, openness, and conscientiousness negatively and significantly predicted the intercept of depressive mood, but insignificantly predicted the slope; and agreeableness did not predict the intercept or slope of depressive mood. Overall, first-year students' depressive mood increased linearly and personality dimensions differently predict depressive mood during this period.