This study focuses on the Paleocene‐Eocene Thermal Maximum (PETM), a hyperthermal event characterized by a rapid increase in global temperature (5–8°C) over 20 ka, in the Southern Pyrenean Foreland Basin. Although there is evidence of increased flood discharge and erosion in the Southern Pyrenees, how paleoclimatic conditions and weathering evolved remains to be assessed. This study focuses on the catchment scale climatological changes recorded in the clay minerals of floodplain paleosols, giving insights into how rainfall affected the weathering regime during and after the hyperthermal event. The oxygen (δ¹⁸O) and hydrogen (δD) isotope compositions were analyzed in two clay fractions in paleosols of the Esplugafreda continental section. The clay minerals comprise a dominant smectite‐rich assemblage, which indicates a predominantly seasonal, semi‐arid climate throughout the section. Two positive excursions in the δ¹⁸O record during the Pre‐Onset Excursion (POE) and the Syn‐PETM support an increase in air near‐surface temperature. Using the δD and δ¹⁸O smectite fractionation factors, we estimate a Mean Annual Air Temperature (MAAT) of 24.2 ± 1.0°C for the POE and 27.0 ± 0.8°C for the Syn‐PETM. The δD values show a relatively stable composition during the climatic disturbance, which suggests that this mid‐latitude catchment was overall characterized by low yearly rainfall, with a peak in extreme events during the body of the PETM and a trend toward aridification during the recovery phase of the PETM, supported by the paleosol morphotype. These climatic conditions suggest a kinetically controlled weathering regime, where physical transport of the sediments played a primary role as a denudation mechanism.