Ramadan fasting can alter hydration, sleep, and training schedules in competitive swimmers aged 6-20 years, but evidence in swimmers is limited. This prospective cohort study investigated within-participant changes in bioelectrical impedance analysis and body composition across Ramadan in swimmers. Swimmers were assessed at 3 time points: approximately 7 days before the first day of Ramadan, during the third week of Ramadan, and approximately 14 days after Ramadan ended. Participants were 52 competitive swimmers aged 6-20 years (39 males). Phase-sensitive single-frequency bioelectrical impedance analysis (50 kHz) measured phase angle (A50) and resistance (R50); derived outcomes included fat-free mass, fat mass, and body mass index. Linear mixed-effects models adjusted for age and sex estimated changes versus baseline with false discovery rate control. R50 increased during (+50 Ω) and after Ramadan (+42 Ω), fat-free mass decreased (-1.86 kg and -1.41 kg), fat mass increased (+1.29 kg and +1.68 kg), A50 declined modestly after Ramadan (-0.38°), and body mass index showed no clear change. Effects were directionally consistent across sex and fasting-type subgroups. Across Ramadan, swimmers aged 6-20 years exhibited higher electrical resistance and unfavorable shifts in body composition despite stable body mass index, consistent with transient hydration/cellular stress and possible tissue-related changes that cannot be confirmed without functional or regional measures. Routine bioelectrical impedance analysis-based monitoring and targeted evening hydration, protein distribution, and light resistance training may help preserve fat-free mass during Ramadan.