Description: A standard CTD system from Sea-Bird Electronics Inc SBE911+ with duplicate temperature and conductivity sensors was used to measure temperature, conductivity and pressure at 81 stations during an expedition to the Kara and Laptev Seas and the adjacent Arctic Ocean in August-September 2021 aboard the research vessel Akademik Tryoshnikov. We followed the recommendation of the manufacturer to calculate salinity with Seabird processing software. The salinity is given as Practical Salinity (PSU). The accuracy of the conductivity sensors was verified by measurements on water samples with a salinometer. The data set published here includes only the data from the first conductivity (SN 3290) and temperature (SN 4127) sensors. Only at station 26 the data of the second sensor pair (SN 2618/Cond, SN 5115/Temp) were used. The CTD was connected to a SBE32 Carousel Water Sampler with 24 12-liter bottles. Additionally, a Benthos Altimeter and a Wetlabs ECO-AFL Fluorometer were connected to the SBE911+ system. At 69 stations, 846 seawater samples were collected for analysis of dissolved inorganic nutrients (nitrate, nitrite, phosphate, ammonium, silicate), oxygen, total alkalinity, and pH. Dissolved inorganic nutrients were analyzed using a segmented flow analyzer from Seal Analytical. Ammonium was measured manually (colorimetric method) using a spectrophotometer (Shimadzu UV-1800). Dissolved oxygen was determined by the standard Winkler titration method using a Metrohm 916 TiTouch automatic titrator and a handheld titrator (BRAND). pH and total alkalinity were measured by potentiometric titration using an automatic titrator (Metrohm 916 TiTouch). The data are provided by the Arctic Century Expedition, a joint initiative led by the Swiss Polar Institute (SPI), the Antarctic and Arctic Research Institute (AARI) and GEOMAR Helmholtz Centre for Ocean Research Kiel (GEOMAR) and funded by the Swiss Polar Foundation, AARI, Minobrnauki (CATS RFMEFI61619X0108) and BMBF (CATS 03F0831).