<p>Feammox, i.e., anaerobic ammonium oxidation coupled with ferric iron (Fe(III)) reduction, has been considered as a promising option for autotrophic nitrogen removal in wastewater with low carbon/nitrogen ratio. This study proposed a novel technology coupling Feammox and vivianite crystallization for the simultaneous removal of ammonium and phosphate in low-strength wastewater. The results showed that the Feammox process achieved high-level phosphate removal with the selective addition of amorphous ferric oxyhydroxide (amorphous FeOOH) as the iron source, compared to the other iron oxides hematite (<i>α</i>-Fe₂O₃) and magnetite (Fe₃O₄). The removal efficiencies of nitrogen and phosphate reached 77.6% and 77.1%, respectively, with the addition of 0.4 g/L amorphous FeOOH (equivalent to 0.3 g Fe/L). Fundamental investigations through a combination of microscopic images, X-ray diffraction, scanning electron microscopy-energy dispersive spectroscopy, Mössbauer spectroscopy and sequential phosphate extraction method revealed potential mechanisms for phosphate removal. With the addition of amorphous FeOOH, phosphate was firstly adsorbed to mineral surface sites, and then formed vivianite crystals along with microbial Fe(III) reduction (i.e., Feammox reaction) for iron availability. As a result, the Fe(II) formed in Feammox served as a phosphate sink through vivianite precipitation. This study improves the understanding of nitrogen and phosphate transformations in the proposed innovative Feammox-based process and supports the development of next-generation wastewater treatment technologies.</p>