Iron deficiency anemia (IDA) is one of the most serious nutritional problems. This study aimed to evaluate the therapeutic effects of a novel agar oligosaccharide–iron complex (AOS–iron) on rats with IDA, such as iron supplementation and recovery of antioxidant ability. Eighty-four weaned male SD rats were randomly divided into a normal control group (n = 12), which was fed with a standard diet, and an anemia model group (n = 72), which was fed with an iron-deficient diet for 4 weeks to establish a model of IDA. After the model was established, the rats with IDA were divided into six groups, namely, an anemia model group, a ferrous gluconate group, a ferrous sulfate (FeSO₄) group, and low-dose (LD), medium-dose (MD) and high-dose (HD) AOS–iron groups, and fed with an iron-deficient diet and different iron supplements for 4 weeks, respectively. The results showed that HD AOS–iron exerted a significant restorative effect by returning blood parameters to normal levels in rats with IDA, including hemoglobin, red blood cells, hematocrit, mean cell volume, mean cell hematocrit, mean cell hemoglobin concentration, serum iron, total iron binding capacity, transferrin saturation, and serum ferritin. A histological analysis suggested that the liver morphology in the MD and HD AOS–iron groups was similar to that in the normal group. Furthermore, MD and HD AOS–iron improved antioxidant activities in the serum and liver. In general, high-dose (the same dose as those of ferrous gluconate and FeSO₄) AOS–iron exhibited the best effects in terms of iron supplementation and antioxidant activities. The present findings showed that AOS–iron might be a potential new iron supplement.