The source of the catastrophic 1908 Messina tsunami, southern Italy, is studied by best-fitting the available datasets of observed runup with a previously published empirical function (i.e., the expected runup distribution). The maximum runup is ∼12 m and was measured ∼30–40 km to the south of the area where the maximum coseismic dislocation and Mercalli–Cancani–Sieberg (MCS) intensities were recorded. The observed runup drops from ∼12 m to less than 1 m in a few tens of kilometers. The comparison between observed and expected runup distributions suggests that the main cause of the 1908 tsunami was a mass failure, thus supporting previously published evidence including tsunami arrival times, bathymetric maps, and chronicles reporting the interruption of submarine cables. This article adds a significant case history to the very limited database of thoroughly documented runup for landslide tsunamis.