Background: Common mental disorders have been found to be related to suicidal ideation and behavior. Research in the field of web-based interventions for common mental disorders, however, usually excludes participants with a suicidal risk, although a large proportion of participants might suffer from suicidal ideation. Aims: To investigate the prevalence of suicidal ideation in common mental disorders in an online sample. Method: In total, 502 participants completed nine web-based questionnaires on common mental disorders, of which 120 were also interviewed by telephone to obtain a diagnosis. Logistic regression analyses were applied to investigate associations between disorders and suicidal ideation. Results: Based on web-based self-report, 53% of participants had some form of suicidal ideation. Fewer participants reported suicidal ideation during the interview by telephone. Depression (multivariate odds ratio 7.1), generalized anxiety disorder (2.1), social phobia (2.1), and posttraumatic stress disorder (1.7) were significantly associated with suicidal ideation, while a higher number of comorbid common mental disorders increased the risk. Conclusion: Researchers and clinicians should be aware that one out of every two helpseekers on the internet with common mental disorders may have suicidal ideation. Comorbidity of two or more disorders greatly increase the risk of suicidal ideation. © 2012 Hogrefe Publishing.