Water temperature data for 6 locations in the Plynlimon site (eLTER site, DEIMS code EXPEER-UK-02), Mid Wales. Recorded with handheld thermometer during routine water sampling for chemical analysis. Plynlimon is at the headwaters of the Rivers Severn and Wye (Hafren and Gwy in Welsh) and currently comprises 9 instrumented catchments. Other Plynlimon hydrochemistry and spatial datasets are available at https://catalogue.ceh.ac.uk/documents/91961a0f-3158-4d00-984d-91eb1e03e8bd. Monitoring at Plynlimon is funded by the Centre for Ecology & Hydrology, and is ongoing since 1968.