Many individuals with obsessive-compulsive disorder (OCD) do not receive professional treatment due to various idiosyncratic barriers. Internet-based cognitive-behavioral therapy (iCBT) is increasingly used to narrow treatment gaps, but the efficacy of such interventions without guidance of therapists has not been well studied. This study evaluated the efficacy of an unguided iCBT that includes third-wave approaches for the treatment of OCD symptoms. A total of 128 individuals with self-reported OCD symptoms were randomly allocated to either an intervention group (unguided iCBT) or to a care-as-usual (CAU) control group following an anonymous baseline assessment via an online survey. Eight weeks after inclusion, a reassessment was carried out online. The Yale-Brown Obsessive-Compulsive Scale served as the primary outcome parameter for detecting symptom changes in the per-protocol sample with at least 60 minutes utilization. The iCBT group showed a significantly stronger reduction of OCD symptoms with a medium effect size (η²p  = 0.06) compared with the control condition. This effect was moderated by the general frequency of Internet usage (η²p  = 0.08); the more time per day users spent online, the less they benefited from the intervention. Secondary outcomes revealed (1) a medium effect size on self-esteem (η²p  = 0.06); (2) no statistically significant effects on quality of life, depression symptoms, impulsivity, or social insecurity; and (3) good acceptability of the intervention. The current study provides evidence that unguided iCBT for OCD may be a viable option for individuals who experience treatment barriers. As non-compliance remains a challenge, this topic needs further research.