Iron oxides are old but gold. Stacks of review articles have been published with regard to sounding the magnificence of iron oxides for many applications. Nevertheless, no existing review papers discuss more specifics on how to fabricate nanometer-sized iron oxides, which, in fact, we can obtain them directly from natural sources and wastes via green technology. Direct synthesis of nanometer-sized iron oxides from natural sources and wastes could simply make them less expensive, and, of course, significantly decrease the energy requirement in the synthesis process, lower time synthesis consumption, and be environmentally benign. Here, we compile and synthesize all information on nanometer-sized iron oxides, starting from their phase diversity and their low-dimensional forms such as in zero-, one-, two-, and three-dimensional (0D, 1D, 2D, and 3D, respectively) shapes. Furthermore, the synthesis of nanometer-sized iron oxides from natural sources and waste materials via green synthesis is explained comprehensively, followed by the elucidation of the prospect and challenges of each synthesis method. Lastly, the recent applications and potential commercialization of nanometer-sized iron oxides synthesized from natural sources and waste materials from the point of view of green technology are also listed in this review article.