On 8 June 2008 a strong (Mw 6.4) earthquake occurred in the area of northwest Peloponnese, western Greece. The event originated in the lower crust and was caused by the rupture of a previously unknown fault probably inherited from past tectonic phases. In this study we perform a stress inversion of all available focal mechanisms in this area in order to obtain an estimate of the regional stress field. It is shown that the maximum principal stress axis has an azimuth of N273°E and forms an angle of 63° with the fault’s strike, which is well-constrained by seismological observations. This implies that the fault was severely misoriented with respect to the prevailing stress field assuming friction coefficients in the range 0.65–0.85. Calculation of pore-fluid factors for a variety of input parameters seems to confirm the presence of elevated fluid pressure near the hypocenter because they reach superhydrostatic to lithostatic values (0.80–1.0). The source of these fluids is probably of deep origin and may have to do with upper mantle degassing. This suggestion is supported by the presence of mantle helium in spring waters close to the epicenter and by low Pn velocities consistent with partially molten mantle beneath northwest Peloponnese.