This systematic review aimed to identify physical performance determinants in competitive youth swimmers, focusing on strength, power, anaerobic, aerobic, and body composition measures. A total of 43 studies met the inclusion criteria, with cross-sectional studies scoring between 4–8 and randomised-controlled trials scoring 8–9 on the Joanna Briggs Institute (JBI) checklist. Youth swimming performance was primarily influenced by muscle strength, muscle power, lean body mass, and anaerobic and aerobic metabolism measures. Improved values in these variables were associated with better swimming performance. Body fat percentage did not show a clear relationship with youth swimming performance. Key findings indicate that greater muscle strength, power, and lean body mass are beneficial for swimming performance, particularly for start and turn performance. Anaerobic and aerobic metabolism measures were important determinants, with middle- and long-distance events more influenced by aerobic capacity. Body fat percentage had a nuanced relationship with swimming performance, requiring further investigation. Findings were inconsistent across studies, potentially due to unidentified confounding factors. Strength and power training are recommended for youth swimmers to enhance performance. Anaerobic power is crucial for sprinting, while aerobic capacity is more important for longer events. Body composition, particularly lean body mass, is positively associated with swimming performance, while body fat percentage shows mixed results. The review highlights the importance of considering multiple physiological factors in youth swimming performance, emphasizing the need for targeted training programs and assessments. Future research should explore the mechanisms linking body composition and swimming performance, as well as optimal levels of lean body mass and body fat percentage for different event distances and strokes.This systematic review aimed to identify physical performance determinants in competitive youth swimmers, focusing on strength, power, anaerobic, aerobic, and body composition measures. A total of 43 studies met the inclusion criteria, with cross-sectional studies scoring between 4–8 and randomised-controlled trials scoring 8–9 on the Joanna Briggs Institute (JBI) checklist. Youth swimming performance was primarily influenced by muscle strength, muscle power, lean body mass, and anaerobic and aerobic metabolism measures. Improved values in these variables were associated with better swimming performance. Body fat percentage did not show a clear relationship with youth swimming performance. Key findings indicate that greater muscle strength, power, and lean body mass are beneficial for swimming performance, particularly for start and turn performance. Anaerobic and aerobic metabolism measures were important determinants, with middle- and long-distance events more influenced by aerobic capacity. Body fat percentage had a nuanced relationship with swimming performance, requiring further investigation. Findings were inconsistent across studies, potentially due to unidentified confounding factors. Strength and power training are recommended for youth swimmers to enhance performance. Anaerobic power is crucial for sprinting, while aerobic capacity is more important for longer events. Body composition, particularly lean body mass, is positively associated with swimming performance, while body fat percentage shows mixed results. The review highlights the importance of considering multiple physiological factors in youth swimming performance, emphasizing the need for targeted training programs and assessments. Future research should explore the mechanisms linking body composition and swimming performance, as well as optimal levels of lean body mass and body fat percentage for different event distances and strokes.