: Creatine monohydrate (CrM) is considered to be one of the most effective supplements for enhancing lean body mass during resistance training. However, CrM may influence body water content, potentially confounding lean body mass measurements. Therefore, this randomised controlled trial assessed the effect of CrM alone on lean body mass following a supplement wash-in, and when combined with a resistance training program. : Sixty-three (34 females, 29 males, 31 ± 8 years) participants were randomised to supplement with CrM (5 g/day for 13 weeks: wash-in + 12-week resistance training) or serve as a control (received no creatine or placebo). Lean body mass was measured using dual X-ray absorptiometry at baseline, post 7-day wash-in, and post 12 weeks of resistance training. Both groups began the same training program post CrM wash-in. : After the 7-day wash-in, the supplement group gained 0.51 ± 1.79 kg more lean body mass than the control group ( = 0.03). Following the wash-in, both groups gained 2 kg after resistance training ( < 0.0001), with no between-group difference in lean body mass growth ( = 0.71). Sex-disaggregated analysis showed that the supplement group, only in females, gained 0.59 ± 1.61 kg more lean body mass than the controls ( = 0.04). There were no group differences in lean body mass growth following resistance training in females ( = 0.10) or males ( = 0.35). : A 7-day CrM wash-in increased lean body mass, particularly in females. Thereafter, CrM did not enhance lean body mass growth when combined with resistance training, likely due to its short-term effects on lean body mass measurements. A maintenance dose of higher than 5 g/day may be necessary to augment lean body mass growth.