BACKGROUND Adjustment disorder is one of the most common mental health diagnoses. Still it receives relatively little attention from researchers trying to establish best interventions to treat it. With high prevalence of stressful life events, which might be leading to adjustment disorder, and limited resources of mental health service providers, online interventions could be a very practical way of helping people who have these disorders or are in the risk to develop them. The proposed study protocol is aimed to describe a randomized controlled trial of an internet-based modular intervention for adjustment disorder as it is defined in a proposal for the ICD-11. METHODS/DESIGN This study is a two-armed Randomized Controlled Trial (RCT) to examine the effectiveness of a web-based intervention BADI (Brief Adjustment Disorder Intervention) for adjustment disorder symptoms. BADI has four modules: Relaxation, Time management, Mindfulness and Strengthening relationships. It is based on stress and coping research and integrates evidence-based treatment approaches such as Cognitive Behavioural therapy (CBT), mindfulness and body-mind practices, as well as exercises for enhancing social support. Primary outcome of the study are symptoms of adjustment disorder and well-being. Engagement into the program and motivation for change is a secondary outcome. All participants after completing the baseline assessment are randomly assigned to one of the two groups: either to the one in which participant will instantly gain access to the BADI intervention or a group in which participants will be given access to the BADI program after waiting one month. Participants of BADI can choose exercises of the program flexibly. There is no particular order in which the exercises should be completed. DISCUSSION Study will provide new insights of modular internet-based interventions efficacy for adjustment disorders. The study will also provide information about the role of motivation and expectancies on engagement in modular internet-based interventions. In case this RCT supports effectiveness of fully automated version of BADI, it could be used very broadly. It could become a cost-effective and accessible intervention for adjustment disorder. TRIAL REGISTRATION The study was retrospectively registered with the Australian and New Zealand Clinical Trials Registry with the registration number ACTRN12616000883415 . Registered 5 July, 2016.