(1) Background: Transcriptomic and proteomic studies provide a wealth of new genes potentially involved in red blood cell (RBC) maturation or implicated in the pathogenesis of anemias, necessitating validation of candidate genes in vivo; (2) Methods: We inactivated one such candidate, transmembrane and coiled-coil domain 2 (Tmcc2) in mice, and analyzed the erythropoietic phenotype by light microscopy, transmission electron microscopy (TEM), and flow cytometry of erythrocytes and erythroid precursors; (3) Results: Tmcc2^−/− pups presented pallor and reduced body weight due to the profound neonatal macrocytic anemia with numerous nucleated RBCs (nRBCs) and occasional multinucleated RBCs. Tmcc2^−/− nRBCs had cytoplasmic intrusions into the nucleus and double membranes. Significantly fewer erythroid cells were enucleated. Adult knockouts were normocytic, mildly polycythemic, with active extramedullary erythropoiesis in the spleen. Altered relative content of different stage CD71⁺TER119⁺ erythroid precursors in the bone marrow indicated a severe defect of erythroid maturation at the polychromatic to orthochromatic transition stage; (4) Conclusions: Tmcc2 is required for normal erythropoiesis in mice. While several phenotypic features resemble congenital dyserythropoietic anemias (CDA) types II, III, and IV, the involvement of TMCC2 in the pathogenesis of CDA in humans remains to be determined.