<p>The eco-friendly synthesis of iron oxide nanoparticles (IONPs) is a notable achievement in nanotechnology, offering a sustainable alternative to conventional physical and chemical methods. Green synthesis utilizes biological entities such as plant extracts, fungi, bacteria and algae, which function as dual reducing and capping agents. This biogenic approach allows for excellent control over size, morphology and crystalline properties of nanomaterials, in contrast to conventional synthesis, which depends on hazardous chemicals and high energy. This review provides a comprehensive analysis of various biogenic synthesis pathways, critically evaluating the underlying molecular mechanisms, Fe²⁺/Fe³⁺ redox chemistry and the thermodynamic and kinetic parameters governing nucleation and crystal growth. We systematically outline the role of advanced analytical techniques (including XRD, TEM, FTIR, DLS and VSM) in characterizing biogenic IONPs with their limitations. Furthermore, this article explores the diverse applications of green-synthesized IONPs across environmental remediation (heavy metal adsorption, photocatalytic dye degradation) and biomedicine (targeted drug delivery, magnetic resonance imaging contrast enhancement, magnetic hyperthermia and antimicrobial action). Finally, we address critical translational hurdles, including the dynamics of the in vivo protein corona, comprehensive systemic toxicity, batch-to-batch variability, Good Manufacturing Practice (GMP) alignment, European Medicines Agency (EMA) and US Food and Drug Administration (FDA) regulatory frameworks necessary to move biogenic IONPs from benchtop discovery to clinical and industrial deployment. This review provides a comprehensive framework for understanding the synthesis, biological interactions and translational potential of green synthesized IONPs toward sustainable nanotechnology and advanced biomedical applications.</p> Graphical abstract <p></p>