BackgroundIron supplementation, especially in female athletes, is one of the influential factors in aerobic capacity, and its deficiency can lead to significant problems related to reduced aerobic capacity. This study aimed to investigate the effect of three weeks of iron supplementation on the aerobic capacity of female handball players. MethodsIn this randomized, double-blinded, and placebo control trial, fourteen elite handball players (age: 21.6 ± 5.68 years; height: 169.5 ± 4.9 cm; weight: 62.2 ± 9.25 kg; BMI: 21.5 ± 2.9 kg/m2) randomly divided into two supplement groups (receiving a 100 mg/day of poly-maltose tri hydroxide iron complex in the form of tablets) and the placebo group (receiving a tablet containing 100 mg/day starch which is the same color and shape as iron tablets). The supplementation protocol was performed for three weeks during the off-season. Maximal oxygen consumption (VO2max), amounts of carbon dioxide at the first ventilatory threshold (VT1 CO2), amounts of carbon dioxide at the second ventilatory threshold (VT2 CO2), Time to exhaustion (TTE), pulmonary ventilation (VE), ventilatory equivalents for oxygen (EQO2), amounts of oxygen at the first ventilatory threshold (VT1 O2), amounts of oxygen at the second ventilatory threshold (VT2 O2), Time to reach first ventilatory threshold (Time VT1), end-tidal partial pressure of oxygen at the first ventilatory threshold (PETO2 VT1), end-tidal partial pressure of carbon dioxide at the first ventilatory threshold (PETCO2 VT1) and ventilatory equivalents for carbon dioxide (EQCO2) were measured using the Bruce test and gas analyzer in two pre-test and post-test stages. ResultThere were significant improvements in VT1 O2, Time VT1, and PETCO2 VT1 and a significant decrease in PETO2 VT1 (P<0.05). Also, no significant changes were found in VO2max, VT1 CO2, VT2 CO2, VT2 O2, TTE, VE, EQO2, and EQCO2 after three weeks of iron supplementation (P>0.05). ConclusionThe study found that three weeks of off-season iron supplementation positively impacted female handball players' aerobic capacity; however, it did not significantly improve their VO2max.