Research expedition SO303 took place from January 23rd, 2024 (La Réunion, France) to February 19th, 2024 (Colombo, Sri Lanka).CTD data for 9 stations with a total of 16 individual casts were recorded using a Sea & Sun Technology CTD90M (SN 979), attached to a HydroBios Multi-Plankton-Sampler down to depths of about 400 m. The CTD was equipped with sensors for pressure (PA7LHE, Keller AG), temperature (Pt100 model 1509, Thermal Developments International), conductivity (7-pole platinum coated electrode cell in quartz glass, Sea & Sun Technology), chlorophyll a (Seapoint Chlorophyll Fluorometer, Seapoint) and photosynthetically active radiation (QSP 2200, Biospherical Instruments Inc.).The data files contain the data for temperature, conductivity, salinity, density, chlorophyll a concentration, sound velocity and photosynthecially active radiation; as raw data and processed and flagged according to the recommendations for real-time data processing of EuroGOOS and GTSPP, as well as the outlier detection method CoTeDe (https://github.com/castelao/CoTeDe). Biogeographic regions are determined according to Spalding et al. (2012). TEOS-10 unit conversions have been performed with the GSW Oceanographic Toolbox (McDougall and Barker, 2011).