Underway-CTD (CTD-UW) data were collected during an September-October 2019 expedition to the Laptev and East Siberian Seas aboard the RV Professor Multanovskiy. The underway CTD manufactured by Ocean Science is a self-contained free-falling probe measuring temperature, conductivity and depth while the ship is transiting. The UCTD was operated 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). In shallower waters (<300 m), the water column was profiled all the way to the seafloor, while in deeper waters, only the upper 600-800 m were sampled. The UCTD was calibrated against a Seabird 9+ CTD during the cruise. Mixing of the water column caused by the ship can reach depths of up to about 5 m, depending on the weather conditions and the ship's speed. Data are from the 2019 Transarktika Expedition (Leg 4/2 from Murmansk to Vladivostok), an initiative of the Russian Antarctic and Arctic Research Institute (AARI), carried out in collaboration with the Russian-German CATS project and funded by Minobrnauki (CATS RFMEFI61619X0108) and BMBF (CATS 03F0831).