Background <p>High-intensity interval training (HIIT) and small-sided games (SSG) are widely used in team-sport conditioning. However, the relative effects of these factors on key physical fitness outcomes in male adolescent team-sport athletes remain unclear.</p> Methods <p>PubMed, Web of Science, Scopus, and Embase were systematically searched for randomised controlled trials directly comparing HIIT and SSG interventions lasting at least 4 weeks in male adolescent team-sport athletes aged 10 to 19 years. Because baseline differences may exist across studies, effect sizes were calculated using pre–post change scores. Random-effects models pooled standardised mean differences with small-sample correction using Hedges’ g, and results are reported with 95% confidence intervals.</p> Results <p>Seventeen randomised controlled trials involving 402 male adolescent team-sport athletes were included, with 12 studies on soccer and 5 on basketball. No significant between-group differences were observed for VO₂max (SMD = 0.05, 95% CI − 0.28 to 0.39, I² = 22.8%), VIFT (SMD = 0.24, 95% CI − 0.08 to 0.56, I² = 0%), CMJ (SMD = 0.02, 95% CI − 0.24 to 0.28, I² = 0%), ≤ 10-m sprint (SMD = − 0.11, 95% CI − 0.38 to 0.16, I² = 0%), ≥ 20-m sprint (SMD = − 0.14, 95% CI − 0.39 to 0.11, I² = 0%), or COD (SMD = 0.00, 95% CI − 0.45 to 0.45, I² = 62.5%). In contrast, HIIT showed a significant advantage over SSG for Yo-Yo IR1 (SMD = 0.42, 95% CI 0.09 to 0.74, I² = 0%). Leave-one-out sensitivity analyses showed that excluding any single study did not materially change the direction or magnitude of the pooled effect sizes.</p> Conclusion <p>In male adolescent team-sport athletes, HIIT and SSG showed comparable improvements in VO₂max, VIFT, CMJ, ≤ 10-m and ≥ 20-m sprint, and COD, whereas HIIT was superior to SSG for Yo-Yo IR1.</p> Registration <p>PROSPERO (CRD420261293288).</p>