Groundwater temperature data from a shallow urban aquifer in Cardiff, Wales, UK between 2018-2024. Monitoring was undertaken as part of the ‘Cardiff Urban Geo-Observatory’ project. Boreholes are located within the urban area of the City of Cardiff, Wales, UK. The majority of temperature sensors were installed within boreholes that monitor a shallow Quaternary aged sand and gravel aquifer, however the made ground and the Triassic Mercia Mudstone also represented. Temperature sensors installed in 54 boreholes, between depths of 1.5m and 18.9 m below ground, with measurements every 30 minutes. The dataset comprises of just over 3.5 million temperature measurements. Monitoring was undertaken by the British Geological Survey and was designed to address knowledge gaps of subsurface Urban Heat Island and its use for heat recovery and storage. Spatial Ref Sys: OSGB 1936 / British National Grid [27700].