Background: A telemedicine care concept based on telephone contacts and individualized text messages was developed for patients with mental disorders to continue treatment after therapy in a psychiatric day hospital. The primary objective of this study was to evaluate the effectiveness of the telemedicine interventions. Methods: The study had a 3-armed, randomized design with 2 intervention arms (intervention 1: telephone contacts; intervention 2: telephone contacts and short text messages; both took place over a period of 6 months and in addition to usual care), and a control group with usual care. Primary outcomes were 18-item Brief Symptom Inventory (BSI-18) scores for anxiety, depression and somatization. All participants were recruited from psychiatric day hospitals. The study was registered in the German Clinical Trials Register (DRKS00000662). Results: 113 participants were analyzed 6 months after starting the intervention. The average BSI-18 anxiety score after 6 months was -2.04 points lower in intervention group 2 than in the control group (p value: 0.042). The difference in BSI depression score between these two groups was marginally significant (p value: 0.1), with an average treatment effect of -1.73. In an exploratory sensitivity analysis restricted to the 75% of patients with the highest symptom scores at baseline, intervention group 1 yielded a significant effect for anxiety and depression compared to the control group (p = 0.036 and 0.046, respectively). Conclusions: Telemedicine provides a novel option in psychiatric ambulatory care with statistically significant effects on anxiety. A positive tendency was observed for depression, especially in cases with higher symptom load at baseline.