Method for evaluating fluid pressures in an underground zone by means of seismic and well data. The zone studied is divided into several time analysis intervals. For each of these intervals, a pre-stack stratigraphic inversion of the seismic data is then carried out using geological a priori information, and a lithoseismic facies cube is determined by lithoseismic analysis. Then, for each facies, relations connecting the seismic impedances of wells to differential pressures in wells are determined in each analysis interval. Finally, a differential pressure cube is determined using the previous relations, as well as a confining pressure cube, in order to determine the fluid pressures in the subsoil by simple difference between these two cubes. Application: petroleum sphere for the detection of overpressure zones during drilling for example.