The earthquake on December 9, 1755 is one of the major events in the Valais region, having an important influence on the seismic hazard assessment of the area. The re-evaluation of the event was undertaken in the course of upgrading the Earthquake Catalog of Switzerland (ECOS). It is now based on a newly investigated and reliable database created through research in archives and libraries. Using the data made available by the historical information, we assigned site intensities according to the criteria established by the European macroseismic scale EMS 98 (Grünthal 1998). Based on the broad spectrum of the available information, we assigned minimum and maximum possible values (Imin; Imax) and determined a most probable intensity value (Iw) for each site. The maximum observed intensity, Ix, is the largest intensity for each quake. Epicentral intensity, I0, is equal to the maximum observed intensity Ix, if at least two data points are assigned an intensity of Ix. We obtained maximum Ix = I0 = VIII at Brig/VS (46.3/8.0) and moment magnitude Mw = 6.1.