<p>The deformability of red blood cells (RBCs) is essential for peripheral circulation and RBC survival and is reportedly altered in several diseases. However, its detail in iron-deficiency (ID) anemia remains poorly understood. We investigated the association between ID and RBC deformability in 120 participants classified into four groups according to their ferritin and hemoglobin levels: non-ID/non-anemia (<i>n</i> = 61), ID/non-anemia (<i>n</i> = 32), ID/anemia (<i>n</i> = 15), and non-ID/anemia (<i>n</i> = 12). In the analysis using the established on-chip deformability checker, the normalized transit velocity of RBCs through the peripheral-vessel constriction models was significantly higher in the ID/anemia group than in the other groups. The RBC deformability index (RDI), derived from the normalized transit velocity and degree of RBC deformation, was highest in the ID/anemia group, followed by the ID/non-anemia group. The RDI was negatively correlated with log₁₀ ferritin levels (<i>r</i> = -0.66, <i>p</i> &lt; 0.01), even after adjusting for hemoglobin levels. Lower log₁₀ ferritin levels correlated with thinner, more oval-shaped RBCs and lower internal viscosity. These RBC characteristics were significantly associated with a higher RDI. These results suggest that, among several known determinants of RBC deformability, RBC morphology and internal viscosity are altered in ID, resulting in higher deformability.</p>