Evidence of self-help interventions for adjustment disorder (AjD) is limited. This study aims at testing in a randomized controlled trial (RCT) the effectiveness of a disorder-specific, Internet-delivered cognitive–behavioural therapy (ICBT) intervention for AjD. Participants were randomly allocated to either an ICBT with brief weekly telephone support (n = 34) or a waiting list group (n = 34). Beck's inventories for depression and anxiety were used as primary outcomes. The secondary outcomes were AjD symptoms, post-traumatic growth, positive and negative affect, and quality of life. In all, 76.5% of the participants completed the intervention. Compared with the control group, participants in the intervention condition showed significantly greater improvement in all outcomes (Cohen's d ranged from 0.54 to 1.21) except in anxiety symptoms measured by Beck Anxiety Inventory (d = 0.27). Only ICBT group showed a significant improvement in post-traumatic growth, positive and negative affect, and quality of life. The number of cases that achieved clinically meaningful change in all outcome measures was also higher in the ICBT group. All therapeutic gains were maintained at 3-, 6- and 12-month follow-ups. The current study provides evidence on the effectiveness of ICBT interventions to reduce the impact of AjD. Results suggest that brief self-help intervention with minimal therapist support is more effective than the mere passage of time in reducing the distress symptoms associated to the disorder and also can confer additional benefits.