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The aim of this study was to examine the relationship between the external load (EL) and internal load among U15, U17 and U19 youth soccer players, and to identify the factors best influencing the Rating of Perceived Exertion (RPE) and session-RPE (s-RPE) from Global Positioning System-derived variables. Data were collected from 50 male youth soccer players over a 14-week in-season period, encompassing a total of 1196 observations (145 training sessions and 33 matches). The findings indicate that during training sessions, the relationship between EL-derived volume variables and s-RPE exhibited moder-ate-to-very-strong correlations (U15—r ranging from 0.23 to 0.52; U17—r ranging from 0.51 to 0.78; U19—r ranging from 0.34 to 0.61, p < 0.001). The strongest relationships were observed with total distance, acceleration, deceleration (DEC) and player load variables (p < 0.001). However, perceived wellness measures showed weak correlations with almost every EL parameter. Considering matches for all age groups, total distance showed moderate to large correlation with s-RPE (ranging from 0.41 to 0.59, p < 0.001). Additionally, RPE and s-RPE were significantly influenced by total distance, acceleration, deceleration, medium-speed running per minute, sprint distance per minute and deceleration per minute variables.