In the South Iceland Seismic Zone (SISZ) present-day tectonic activity is mainly associated with a conjugate system of NNE-trending right-lateral strike-slip faults and ENE-trending left-lateral ones ; NW-trending faults are also present. All these faults affect basaltic lavas and hyaloclastites, Upper Tertiary-Pleistocene to Post-Glacial in age, in the SISZ where we collected about 700 brittle tectonic data at 25 sites. Most sites provided inhomogeneous data sets related to extensional as well as strike-slip regimes. At each site, the whole set cannot be accounted for by a single tectonic stress regime. It rather reveals two opposite stress regimes, each including normal and strike-slip faulting. The fault slip data subsets were distinguished on the basis of mechanical consistency. The analysis of the strike-slip fault in the major data subset indicates paleostress tensors with subhorizontal and axes, the trend of being N270°E to N340°E (main trend: N315°E). The normal faults in the major subset characterize vertical and horizontal also trending N270°E to N340°E (mainly N310°E). In the minor data subsets, the direction of , for the strike-slip faults as well as the normal ones, ranges from N20°E to N80°E (N45°E on average). The major stress regime, a NW-SE trending extension, includes 70% of the total population of faults, whereas the minor one, a NE-SW trending extension, includes 30%. Of a total of 718 fault slip data, 55% indicate primarily normal-slip and 45% primarily strike-slip. The ratio normal/strike-slip faults is lower than in other areas in Iceland (e.g. , the Vestfirdir peninsula where strike-slip faults represent 20% of the total population). Moreover, many normal faults were generated in the western rift-zone segment, then drifted out of it. Some of them were reactivated later, when located outside the rift zone. The above results indicate that the dominating stress field in the SISZ favours strike-slip faulting, with an horizontal axis trending approximately WNW-ESE to NW-SE. We point out, however, that in addition, there is a contrasting minor, stress field, characterized by approximately NNE-SSW to NE-SW extension. These results were compared to the stress regimes determined from earthquake focal mechanisms (see part II : Angelier and Bergerat). This comparison reveals that the paleostress and stress regimes are identical and that the changes in the stress field of the SISZ are characterized by stress permutations between and and between and .