Microalgae are potential iron supplements for improving iron deficiency through an unknown mechanism. To analyze the increase in non-heme iron absorption caused by microalgae, six different microalgal feeds were prepared from Spirulina, Chlorella and Synechococcus sp. PCC 7002 as the main source of dietary iron (25 mg kg⁻¹; denoted as H-Sp, H-Ch, and H-Sy, respectively) or as a partial source of dietary iron (5 mg kg⁻¹; denoted as L-Sp, L-Ch, and L-Sy, respectively) to suppress iron-deficiency anemia in rats. The hemoglobin regeneration efficiencies in anemic rats were in the order ferric citrate (34.7 ± 1.8%) < H-Ch (49.9 ± 4.1%) ≈ H-Sy (50.6 ± 5.3%) ≈ L-Sp (46.9 ± 6.2%) ≈ L-Ch (43.1 ± 6.9%) ≈ L-Sy (43.5 ± 2.4%) ≈ FeSO₄ (47.2 ± 4.9%) < H-Sp (54.8 ± 5.5%). The percentage content of intestinal nanosized iron in the H-Sp, H-Ch, and H-Sy treatment groups was significantly higher than that in the L-Sp, L-Ch, and L-Sy groups, and was significantly higher in the microalgal diet groups than in the ferric citrate group, providing strong evidence for nanosized iron supplementation from microalgae. Overall, microalgae, especially Spirulina, are functional iron nutritive fortifiers that can supply intestinal nanosized iron.