The authors compared the effectiveness of two modes of daily iron supplementation in athletes with suboptimal iron stores: oral iron (PILL) versus transdermal iron (PATCH). Endurance-trained runners (nine males and 20 females), with serum ferritin concentrations <50 μg/L, supplemented with oral iron or iron patches for 8 weeks, in a parallel group study design. Serum ferritin was measured at baseline and fortnightly intervals. Hemoglobin mass and maximal oxygen consumption ($\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$) were measured preintervention and postintervention in PATCH. A linear mixed effects model was used to assess the effectiveness of each mode of supplementation on sFer. A repeated-measures analysis of variance was used to assess hemoglobin mass and $\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ outcomes in PATCH. There was a significant time effect (p < .001), sex effect (p = .013), and Time × Group interaction (p = .009) for sFer. At Week 6, PILL had significantly greater sFer compared with PATCH (15.27 μg/L greater in PILL; p = .019). Serum ferritin was 15.53 μg/L greater overall in males compared with females (p = .013). There were no significant differences in hemoglobin mass (p = .727) or $\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ (p = .929) preintervention to postintervention in PATCH. Finally, there were six complaints of severe gastrointestinal side effects in PILL and none in PATCH. Therefore, this study concluded that PILL effectively increased sFer in athletes with suboptimal iron stores, whereas PATCH showed no beneficial effects.