We explored the appropriateness and measurement properties of a suicidality assessment tool (SBQ-R) developed for the general population, in autistic adults--a high risk group for suicide. 188 autistic adults and 183 general population adults completed the tool online, and a sub-sample (n = 15) were interviewed while completing the tool. Multi-group factorial invariance analysis of the online survey data found evidence for metric non-invariance of the SBQ-R, particularly for items three (communication of suicidal intent) and four (likelihood of suicide attempt in the future). Cognitive interviews revealed that autistic adults did not interpret these items as intended by the tool designers. Results suggest autistic adults interpret key questions regarding suicide risk differently to the general population. Future research must adapt tools to better capture suicidality in autistic adults.