The October 11, 1918 ML 7.5 earthquake in the Mona Passage between Hispaniola and Puerto Rico generated a local tsunami that claimed approximately 100 lives along the western coast of Puerto Rico. The area affected by this tsunami is now significantly more populated. Newly acquired high-resolution bathymetry and seismic reflection lines in the Mona Passage show a fresh submarine landslide 15 km northwest of Rinćon in northwestern Puerto Rico and in the vicinity of the first published earthquake epicenter. The landslide area is approximately 76 km2 and probably displaced a total volume of 10 km3. The landslide's headscarp is at a water depth of 1200 m, with the debris flow extending to a water depth of 4200 m. Submarine telegraph cables were reported cut by a landslide in this area following the earthquake, further suggesting that the landslide was the result of the October 11, 1918 earthquake. On the other hand, the location of the previously suggested source of the 1918 tsunami, a normal fault along the east wall of Mona Rift, does not show recent seafloor rupture. Using the extended, weakly non-linear hydrodynamic equations implemented in the program COULWAVE, we modeled the tsunami as generated by a landslide with a duration of 325 s (corresponding to an average speed of ~ 27 m/s) and with the observed dimensions and location. Calculated marigrams show a leading depression wave followed by a maximum positive amplitude in agreement with the reported polarity, relative amplitudes, and arrival times. Our results suggest this newly-identified landslide, which was likely triggered by the 1918 earthquake, was the primary cause of the October 11, 1918 tsunami and not the earthquake itself. Results from this study should be useful to help discern poorly constrained tsunami sources in other case studies.