Background: Internet-based cognitive therapy with monitoring via text messages (mobile CT), in addition to treatment as usual (TAU), might offer a cost-effective way to treat recurrent depression. Method: Remitted patients with at least 2 previous episodes of depression were randomized to mobile CT in addition to TAU (n = 126) or TAU only (n = 113). A linear mixed model was used to examine the effect of the treatment condition on a 3-month course of depressive symptoms after remission. Both an intention-to-treat analysis (n = 239) and a completer analysis (n = 193) were used. Depressive symptoms were assessed using the Inventory of Depressive Symptomatology (IDS-SR₃₀) at baseline and 1.5 and 3 months after randomization. Results: Residual depressive symptoms showed a small but statistically significant decrease in the intention-to-treat group over 3 months in the mobile CT group relative to the TAU group (difference: -1.60 points on the IDS-SR₃₀ per month, 95% CI = -2.64 to -0.56, p = 0.003). The effect of the treatment condition on the depressive symptomatology at the 3-month follow-up was small to moderate (Cohen's d = 0.44). All analyses among completers (≥5 modules) showed more pronounced treatment effects. Adjustment for unequally distributed variables did not markedly affect the results. Conclusions: Residual depressive symptoms after remission showed a more favorable course over 3 months in the mobile CT group compared to the TAU group. These results are a first indication that mobile CT in addition to TAU is effective in treating recurrently depressed patients in remission. However, demonstration of its long-term effectiveness and replication remains necessary.