On 20 July 2017, an M_w6.6 earthquake occurred offshore Kos Island, the largest to occur in the affected area in the instrumental era, and in the past 60 years in the southeastern Aegean Sea. We estimated the aftershocks relative locations by applying the double-difference technique using both differential times from phase-picked data and waveform cross-correlation. The relocated aftershocks are clustered at least in three distinctive patches, creating a zone getting a total length of about 40 km, elongated in a nearly east-west direction, mainly concentrated at depths 8–15 km, with the mainshock hypocenter placed at ~13 km, implying a seismogenic layer of 7 km thickness, indicative for normal faulting earthquakes with M_max~6.5. The aftershock fault plane solutions are predominantly suggestive of normal faulting in response to the north-south extension of the back-arc Aegean area. We further applied the satellite radar interferometry (InSAR) technique to define the coseismic surface displacements. This field of deformation along with the available vectors of displacement measured by the Global Navigation Satellite System (GNSS) technique was combined with the seismological data to determine the rupture geometry and process, with the coseismic slip ranging between 0.5 and 2.3 m. The peak moment release occurred in the depth interval of 9–11 km, consistent with the depth distribution of seismicity in the study area. We used the variable slip model to calculate Coulomb stress changes and investigate possible triggering due to stress transfer to the nearby fault segments.