Anaerobic ammonium oxidation coupled to nitrite reduction (termed as Anammox) was demonstrated as an efficient pathway to remove nitrogen from a wastewater treatment system. Recently, anaerobic ammonium oxidation was also identified to be linked to iron(III) reduction (termed Feammox) with dinitrogen, nitrite, or nitrate as end-product, reporting to enhance nitrogen removal from the wastewater treatment system. However, little is known about the role of Anammox bacteria in the Feammox process. Here, slurry from wastewater reactor amended with ferrihydrite was employed to investigate activity of Anammox bacteria in the Feammox process using the ¹⁵N isotopic tracing technique combined with 16S rRNA gene amplicon sequencing. A significantly positive relationship between rates of ¹⁵N₂ production and iron(III) reduction indicated the occurrence of Feammox during incubation. Relative abundances of Anammox bacteria including Brocadia, Kuenenia, Jettenia, and unclassified Brocadiaceae were detected with low relative abundances, whereas Geobacteraceae dominated in the treatment throughout the incubation. ¹⁵N₂ production rates significantly positively correlated with relative abundances of Geobacter, unclassified Geobacteraceae, and Anammox bacteria, revealing their contribution to nitrogen generation via Feammox. Overall, these findings suggested Anammox bacteria or cooperation between Anammox bacteria and iron(III) reducers serves a potential role in Feammox process.