Same-sex attracted young adults have been found to experience higher rates of mental health problems and greater difficulties in accessing specialist mental health care services compared to their heterosexual peers. Internet-based mental health interventions have the potential to be more engaging and accessible to young adults compared to those delivered face-to-face. However, they are rarely inclusive of lesbian women and gay men. Thus, the current study aims to evaluate the effectiveness of an online mental health and wellbeing program, Out & Online (http://www.outandonline.org.au), in comparison to a wait-list control group, for reducing anxiety and depressive symptoms in same-sex attracted young adults aged between 18 and 25 years. Methods/Design: We are recruiting, through media and community organisations, 200 same-sex attracted young adults with anxiety and/or depressive symptoms and mild to moderate psychological distress (Kessler-10 score between 16 to 21). Participants will be randomly allocated to the intervention (the online program) or the wait-list control group based on a permuted blocked randomisation method to allow for stratification by gender. Participants in the intervention group will receive a tailored program for up to three types of mental health difficulties simultaneously. The primary outcome of anxiety and/or depressive symptoms, and secondary outcomes related to psychological distress, wellbeing and health behaviour will be measured at pre-intervention (0 week), post-intervention (8 weeks) and at a 3-month follow-up (20 weeks). Discussion: This online mental health and wellbeing program will be one of the first online interventions to be designed specifically to be relevant for same-sex attracted individuals. If the program is found to be effective it will improve access to specialist same-sex attracted-relevant mental health services for young adults and will facilitate wellbeing outcomes for these individuals. This program will also be a significant development in the delivery of tailored interventions that target multiple types of mental health conditions simultaneously.