An important element of understanding the genotype–phenotype link in psychiatric disorders lies in identifying the psychological mechanisms through which genetic variation impacts mental health. Here we examined whether emotional inertia, the tendency for a person’s emotions to carry over from 1 moment to the next and a prospective predictor of the development of depression, is associated with a known genetic risk factor for emotional dysregulation, a polymorphism in the serotonin transporter gene (5-HTTLPR). Two hundred thirty-six adolescents recorded their positive and negative emotions in daily life 9 times a day for 6 consecutive days using smartphones, completed a depression questionnaire, and were genotyped for the 5-HTTLPR polymorphism. Carriers of the short 5-HTTLPR were characterized by higher inertia for negative emotions, even after controlling for depressive symptoms. These findings suggest a possible psychological pathway how the serotonin transporter gene contributes to risk for depression.