This study examined mineral transformations during anoxic bioreduction of iron hydroxide and iron oxyhydroxysulfate found in acid mine drainage (AMD) into iron sulfide (FeS) and siderite (FeCO3) nanoparticles. Glucose (10 mM) was inoculated into AMD to stimulate indigenous bacterial growth for bioreduction of Fe(III)-containing minerals. Changes in microbial, geochemical, and mineralogical characteristics were monitored via 16S rRNA, XRD, SEM-EDX, TEM-EDX, ICP-AES, and IC analyses. The AMD was found to be rich in elements, including Fe, Al, Mn, Na, and S (SO4), and had a pH of 5.2. The mineral contents mainly consisted of Fe(III)-containing minerals, such as schwertmannite [Fe8O8 (OH)8-2x(SO4)x · nH2O] and akaganeite [β-FeO(OH)]. During anoxic bioreduction of AMD, the Fe(III)-containing minerals were transformed by indigenous iron-reducing bacteria (e.g., Geobactersp.) into Fe(II)-containing minerals, such as iron sulfide (FeS) and iron carbonate, siderite (FeCO3), within 3-4 days. The microbially-formed iron sulfide (FeS) and siderite (FeCO3) were of 40-60 nm and 10 nm-3 µm in size, respectively. These results not only show that indigenous iron-reducing bacteria in AMD can aid or accelerate formation of Fe(II)-containing minerals when under anoxic environments, but can also offer a simple method for microbial synthesis of nano-sized Fe(II)-containing minerals that can be used as catalysts for environmental remediation by recycling AMD.