A sustainable and environmentally friendly approach to nanotechnology is the green synthesis of gold nanoparticles (AuNPs) using microorganisms. The intracellular and extracellular reduction of gold ions (Au3⁺) into stable metallic gold (Au⁰) is achieved by bacteria, fungi, and marine microbes. Bacterial strains such as Bacillus subtilis and Shewanella oneidensis use enzymatic reduction to generate highly stable nanoparticles, which find applications in biomedical and environmental domains. Fungi such as Aspergillus niger and Fusarium oxysporum produce extracellular enzymes that facilitate the formation of nanoparticles with enhanced stability and biocompatibility. Marine organisms, adaptable to extreme conditons, including Marinobacter hydrocarbonoclasticus, have been employed for large-scale, cost-effective production. This method does not involve the use of toxic chemicals in the process, thereby making it safe for use in medicine, catalysis, and environmental applications. Despite the challenges of purification and standardization, microbial synthesis can be considered a scalable and sustainable alternative to chemical methods. This chapter will highlight the mechanisms, advantages, and industrial potential of microorganism-mediated AuNP synthesis and how it can be used to advance green nanotechnology as well as sustainable materials science.