Stoichiometric Ca2CuO3, having square-planar Cu-chains [C. L. Teske and H. Müller-Buschbaum, Z. Anorg. Allg. Chem. 379, 234 (1970); M. Hjorth and J. Hyldtoft, Acta Chem. Scand. 44, 516 (1990)], is expectedly antiferromagnetic due to 1D intrachain superexchange [K. Okuda, S. Noguchi, K. Konishi, H. Deguchi, and K. Takeda, J. Magn. Magn. Mater. 104–107, 817 (1992)]. Nonetheless, we report remarkable spontaneous ferromagnetism at 293 K after sintering, prominently in oxygen. This apparently introduced excess oxygen at vacant quasioctahedral sites, promoting spin-flip and ferromagnetic interchain coupling. Thermogravimetry (TGA) revealed excess oxygen, ≈0.17 O/Cu. X-ray diffraction (XRD) yielded a comparatively smaller unit cell. Ferromagnetism disappeared by oxygen depletion, mimicking reported nonmagnetism of Ca2CuO3−δ [Okuda et al. (1992)]. Elemental analysis showed insignificant magnetic impurity traces. Tc ranged between 680 K and 723 K, depending on freshness and purity. Saturation magnetization varied with processing, optimally 0.30 A · m2/kg at 1.0 T applied field. Coercivity and remanence varied with purity.