BACKGROUND: Previous work has found that combining plyometric and resistance training in a single session can be effective in improving maximum strength (muscle contraction ability), vertical jumping, and sprint acceleration performance in young soccer players. However, the literature is scarce in relation to young rugby union players. Thus, the purposes of the present study were to examine the effects of a 6-week combined training (CT) programme that combined plyometric and resistance training in the same session on physical performance measures in adolescent male rugby union players and whether a 4-week period of plyometric training exclusion (Detraining) affects training adaptations. METHODS: The participants ( = 15) completed a 6-week CT intervention and 4 weeks of plyometric detraining during the schoolboy rugby union in-season. A performance testing battery was conducted pre-intervention, post-intervention, and 4 weeks post-intervention. RESULTS: A repeated measures ANOVA showed a significant effect for time on CMJ variables (mean power, mean force, and modified reactive strength index [RSI]), 3RM back squat strength, and 505 test time ( < 0.05). Pairwise comparisons revealed that CMJ variables (mean force, mean power, and peak power), 3RM back squat strength, and 505 test performance significantly increased from pre-intervention to post-intervention ( < 0.05). The 4-week plyometric detraining period significantly augmented CMJ variables (mean force, mean power, and modified RSI) and performance measures (standing broad jump [SBJ], 20 m sprint time, 505 test, and 3RM strength). CONCLUSIONS: The findings indicate that a 6-week CT programme can augment measures of lower-body power, maximal strength, and change of direction performance in adolescent male rugby union players, and a 4-week resistance training-only period (plyometric detraining) does not negatively affect performance adaptations.