Underway-CTD (UCTD) data were collected during an August-September 2021 expedition to the Arctic Ocean aboard the RV Akademik Tryoshnikov. The underway CTD manufactured by Ocean Science is a self-contained free-falling probe measuring temperature, conductivity, depth while the ship is transiting. The UCTD was operated mostly in yoyo-mode during selected transects on the shelf of the Kara and Laptev Seas and between the shelf and the basin across the continental slope of the Eurasian Basin while the ship was transiting with 4 - 10 knots. The UCTD probe records the start time of the measurements and stores 16 samples each second internally. The exact location of each profile was subsequently found based on the time stamp from the cruise track. The unpumped conductivity sensor has a slower response time than the temperature sensor, which makes the computation of salinity from conductivity and temperature potentially spiky, especially in the pycnocline or in frontal regions. We followed the recommendation of the manufacturer to calculate salinity with Seabird processing software. The salinity is given as Practical Salinity (PSU). The raw data profiles are provided as well and can be reprocessed if desired. In shallower waters (<300 m), the water column was profiled all the way to the seafloor, while in deeper waters, only the upper 300-600 m were sampled. The UCTD was calibrated against a Seabird 9+ CTD during the cruise. The upper 5 m of the data were discarded due to the influence of the ship. The icebreaker Akademik Tyoshnikov has a deep draft. Mixing of the water column caused by the ship can reach depths of up to about 10 m, depending on the weather conditions and the ship's speed. 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)