We carried out monitoring of surface electric potential variations at the Soultz Hot Dry Rock site (France) during a stimulation experiment where 23 000 m3 of water were injected in granite at 5 km depth. We have observed a long-period (9--10 days) potential variation with a maximum amplitude of ≈5 mV, thanks to the conductive well casing. The start of this anomalous potential coincides with that of the water injection and its magnitude increases as more water is injected before returning toward its original value after shut-in. We interpret this anomalous potential as an electrokinetic effect which is caused by the injection of water circulating in deep fractures. We modeled the pressure distribution during the stimulation and obtained the surface SP using electrokinetic coupling relationships. We found that an in-situ coupling coefficient of ≈-1 mV/MPa, at the low end of laboratory values, gives a good fit to the data.