On 28 September 2018, Indonesia was struck by an 7.5 strike‐slip earthquake. An unexpected tsunami followed, inundating nearby coastlines leading to extensive damage. Given the traditionally non‐tsunamigenic mechanism, it is important to ascertain if the source of the tsunami is indeed from coseismic deformation, or something else, such as shaking induced landsliding. Here we determine the leading cause of the tsunami is a complex combination of both. We constrain the coseismic slip from the earthquake using static offsets from geodetic observations and validate the resultant “coseismic‐only” tsunami to observations from tide gauge and survey data. This model alone, although fitting some localized run‐up measurements, overall fails to reproduce both the timing and scale of the tsunami. We also model coastal collapses identified through rapidly acquired satellite imagery and video footage as well as explore the possibility of submarine landsliding using tsunami raytracing. The tsunami model results from the landslide sources, in conjunction with the coseismic‐generated tsunami, show a greatly improved fit to both tide gauge and field survey data. Our results highlight a case of a damaging tsunami the source of which is a complex mix of coseismic deformation and landsliding. Tsunamis of this nature are difficult to provide warning for and are underrepresented in regional tsunami hazard analysis.