During research cruise POS538, 2D seismic reflection data were collected onboard research vessel FS POSEIDON in November 2019. The profiles cover the Santorini caldera and the neighboring basins around Santorini. Some profiles were used as side survey data for IODP expedition 398 and will only be available after the IODP embargo period. The dataset consists of SEGY files with UTM coordinates stored in bytes 73 and 77 and shot points stored in byte 5. The coordinate system is UTM35 WGS84. Multi-channel seismic data of cruise POS538 were acquired with a high lateral resolution (CMP spacing of ~1.56 m). As a seismic source, we used a GI-pulser that was operated in harmonic mode with primary and secondary volumes of 45 in3. Seismic energy was recorded by multiple concatenated Geometrics GeoEel streamer segments, resulting in active streamer sections ranging from 190 m to 250 m in length. Processing comprised trace-editing, simple frequency filtering (15-1500 Hz), and multiple suppression by means of surface-related multiple elimination (SRME). This was followed by spherical divergence correction, time-variant frequency filtering, pre-stack time migration, top-muting, and white-noise removal. With a main frequency of 125 Hz, the vertical resolution can be approximated to approx. 4-8 m. For more information, see the cruise report (doi:10.3289/cr_pos538).