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AgNbO₃ is one of the prominent lead‐free antiferroelectric (AFE) oxides, which readily exhibits a field‐induced AFE to ferroelectric phase transition and thus a high energy storage density. The solid‐state synthesis of AgNbO₃ is considered difficult and an oxidizing atmosphere is typically employed during AgNbO₃ processing, on the premise that oxygen can prevent possible decomposition of the silver oxide at high temperatures. However, details about the influence of processing parameters on the functional properties of AFE AgNbO₃ are insufficiently understood. In this work, the solid‐state reaction of a stoichiometric AgO and Nb₂O₅ mixture was investigated. We found that ball milling can convert AgO into metallic Ag, which is beneficial for lowering the reaction temperature for the formation of the perovskite phase to 500‒600℃. Moreover, the influence of the processing atmosphere (air, O₂, and N₂) was investigated by thermal analysis and in situ X‐ray diffraction. Since the reaction between Ag and Nb₂O₅ to form AgNbO₃ requires oxygen uptake, AgNbO₃ was only found to form in air and O₂, whereby the kinetics were faster in the latter case. All the sintered AgNbO₃ samples exhibited a similar crystallographic structure, although the samples processed in O₂ had a lower oxygen vacancy concentration. Despite this, well‐defined AFE double polarization loops were obtained in all cases. Our results indicate that decomposition of sliver oxide during ball milling is beneficial for the solid‐state reaction, while a pure O₂ atmosphere is not essential for the synthesis of high‐quality AgNbO₃. These findings may simplify the processing and facilitate further research of AgNbO₃‐based antiferroelectrics.