The Frith-Happé Animations Test, depicting interactions between triangles, is widely used to measure theory of mind (ToM) ability in autism spectrum disorder (ASD). This test began with recording, transcribing, and subjectively scoring participants' verbal descriptions, which consistently found ToM-specific difficulties in ASD. More recently in 2011, White et al. created a more objective version of this ToM test using multiple-choice questions. However, there has been surprisingly little uptake of this test, hence it is currently unclear if White et al.'s findings replicate. Further, the lack of an online version of the test may be hampering its use in large-scale studies and outside of research settings. Addressing these issues, we report the development of a web-based version of the Frith-Happé Animations Test for autistic and neurotypical adults. An online version of the test was developed in a large general population sample (study 1; N = 285) and online data were compared with those collected in a lab-based setting (study 2; N = 339). The new online test was then administered to adults with a clinical diagnosis of ASD and matched neurotypical controls (study 3; N = 231). Results demonstrated that the test could successfully be administered online to autistic adults, who showed ToM difficulties compared to neurotypical adults, replicating White et al.'s findings. Overall, we have developed a quicker, more objective, and web-based version of the Frith-Happé Animations Test that will be useful for social cognition research within and beyond the field of autism, with potential utility for clinical settings. LAY SUMMARY: Many autistic people find it hard to understand what other people are thinking. There are many tests for this 'mentalising' ability, but they often take a long time to complete and cannot be used outside of research settings. In 2011, scientists used short silent animations of moving shapes to create a fast way to measure mentalising ability. We developed this into an online test to use in research and clinics to measure mentalising ability in autism.