The Cinarcik Basin is a transtensional basin located along the northern branch of the northern North Anatolian Fault (NAF) in the Sea of Marmara, the eastern half of which has been identified as a seismic gap. During the SEISMARMARA (2001) experiment, a dense grid of multichannel seismic reflection profiles was shot, covering the whole Cinarcik Basin and its margins. The new seismic images provide a nearly three-dimensional view of the architecture of the basin (fault system at depth and sedimentary infill) and provide insight into its tectonic evolution. Along both northern and southern margins of the basin, seismic reflection data show deep-penetrating faults, hence long-lived features, which have accommodated a large amount of extension. There is no indication in the data for a single throughgoing strike-slip fault, neither a cross-basin fault nor a pure strike-slip fault running along the northern margin. Faster opening is presently observed in the eastern part of the basin. The Cinarcik Basin seems to have developed as a transtensional basin across strike-slip segments of the northern NAF for the last few million years.