The dataset comprises of vertical profiles of water column properties (temperature, salinity, conductivity, pressure, depth, sound velocity and density) of the upper ocean (up to 500m) from 6 locations around St Helena Island. The locations were repeat sampled monthly over 22 months between December 2017 and September 2019, with opportunistic profiles taken from the two main seamounts (Bonaparte and Cardno complex) and the surrounding open ocean. Each individual cast, with up and down profiles, is contained in a separate .csv file. The file naming follows this schema: YYYYMM_stationname. An additional .csv file is provided with details of each serial number and CTD cast (with date, station names also included). Data have been gathered using a Valeport mini CTD (with a passive flow induction cell, Platinum Resistance Thermometer and a strain gauge transducer) and extracted using Valeport DataLogX2 software (version1.0.4.1270). The CTDs were deployed using vertical deployment from a 12m vessel and were hand hauled. No calibration has been applied when using the Valeport mini-CTD with acetal housing and have been supplied to BODC in raw format. The surveyors have been dependent on the sensor calibrations applied by the CTD manufacturer (Valeport). The data have been collected in the framework of the Darwin PLUS Initiative project 'Oceanographic influences on the St Helena pelagic ecosystem' (reference number 070). The data collection activities were carried out by the Government of St Helena with additional collaboration from the British Antarctic Survey (BAS), the Centre for Environment, Fisheries and Aquaculture Science (Cefas), and the International Commission for the Conservation of Atlantic Tunas (ICCAT). As part of the blue-belt initiative, St Helena was planning to declare a Category VI Sustainable use MPA in the entire maritime zone in early 2017. A key-part of ensuring sustainability is to understand the pelagic ecosystem and how seasonal or long-term changes in that system will impact the abundance and distribution of the whale sharks and fish on which the economy of the island depends. The Darwin project wanted to address key priorities including (i) improving conservation and management of the marine environment (ii) developing ecosystem approaches to marine management (iii) promoting sustainable fisheries and (iv) developing data systems on biodiversity.