While suicide prevention efforts are increasingly being delivered using technology, no scales have been developed specifically for web-based use. The Suicidal Ideation Attributes Scale (SIDAS) was developed and validated as a brief, web-based measure for severity of suicidal ideation, using an online survey of Australian adults (n = 1,352). The SIDAS demonstrated high internal consistency and good convergent validity. Frequency and controllability of thoughts were more strongly associated with suicide plans and attempts than other attributes assessed. Scores ≥ 21 indicated high risk of suicide behavior. The SIDAS appears to be a valid web-based measure for severity of suicidal ideation. © 2014 The American Association of Suicidology.