Second-year sea-ice thickness, draft, salinity, temperature, nutrient concentrations ([NO3]- + [NO2]-; [NO2]-, [NH4]+, [PO4]-, Si(OH)4), stable water isotope composition and density were measured during near-weekly surveys at the main second-year ice coring site (MCS-SYI) during the MOSAiC expedition (legs 1 to 3) and new second-year ice coring site leg 4, since the earlier site was not accessible any longer. The ice cores were extracted either with a 9-cm (Mark II) or 7.25-cm (Mark III) internal diameter ice corers (Kovacs Enterprise, US). This data set includes data from 18 coring site visits and were performed from 28 October 2019 to 20 July 2020 at coring locations within 50 m to each other in the MOSAiC Central Observatory. During each coring event, ice temperature was measured in situ from a separate temperature core, using Testo 720 thermometers in drill holes with a length of half-core-diameter at 5-cm vertical resolution. Ice bulk practical salinity was measured using a YSI 30 conductivity meter. Nutrient analyses were conducted shipboard or samples were frozen and analyzed at a shore-based lab. In either case, analyses were conducted colorimetrically using a Seal Analytical AA3 continuous flow auto analyzer (AACE Software, Version 7.09). Measurement of nutrients followed best practices adopted from GO-SHIP recommendations (Hydes et al., 2010; Becker et al., 2020). Ice density was measured using the hydrostatic weighing method (Pustogvar and Kulyakhtin, 2016) from a density core in the freezer laboratory onboard Polarstern at the temperature of –15°C. Relative volumes of brine and gas were estimated from ice salinity, temperature and density using Cox and Weeks (1983) for cold ice and Leppäranta and Manninen (1988) for ice warmer than –2°C. The data contains the event label (column 1), date/time (2), and global coordinates (3,4) of each coring activity. Each salinity/isotope/nutrient core has its manually measured ice thickness (5), ice draft (6), core length (7), and mean snow height (36). Each core section has the total length of its top (8) and bottom (9) measured in situ, as well estimated depth of section top (10), bottom (11), and middle (12). The depth estimates assume that the total length of all core sections is equal to the measured ice thickness. Each core section has the value of its practical salinity (13), nitrate + nitrite (plus error and quality flag; 14, 15, 16), nitrite (17, 18, 19), ammonium (20, 21, 22), phosphate (23, 24, 25), silicate (26, 27, 28), stable water isotopic sample ID and values (29, 30, 31, 32) (Meyer et al., 2000), as well as sea ice temperature (33) and ice density (34) interpolated to the depth of salinity measurements. The global coordinates of coring sites were measured directly. When it was not possible, coordinates of the nearby temperature buoy 2019T62 (legs 1-3) or 2019T61 (leg 4) were used. Ice mass balance buoy 2019T62 installation is described in doi:10.1594/PANGAEA.940231, ice mass balance buoy 2020T61 installation is described in doi:10.1594/PANGAEA.926580. Brine volume (35) fraction estimates are presented only for fraction values from 0 to 30%. Each core section also has comments (37) describing if the sample is from a new coring site or has any other special characteristics.