A novel Fe₂Mo₃O₈/MoO₂@MoS₂ nanocomposite is synthesized for extremely sensitive detection of NH₃ in the breath of kidney disease patients at room temperature. Compared to MoS₂, α‐Fe₂O₃/MoS₂, and MoO₂@MoS₂, it shows the optimal gas‐sensing performance by optimizing the formation of Fe₂Mo₃O₈ at 900 °C. The annealed Fe₂Mo₃O₈/MoO₂@MoS₂ nanocomposite (Fe₂Mo₃O₈/MoO₂@MoS₂‐900 °C) sensor demonstrates a remarkably high selectivity of NH₃ with a response of 875% to 30 ppm NH₃ and an ultralow detection limit of 3.7 ppb. This sensor demonstrates excellent linearity, repeatability, and long‐term stability. Furthermore, it effectively differentiates between patients at varying stages of kidney disease through quantitative NH₃ measurements. The sensing mechanism is elucidated through the analysis of alterations in X‐ray photoelectron spectroscopy (XPS) signals, which is supported by density functional theory (DFT) calculations illustrating the NH₃ adsorption and oxidation pathways and their effects on charge transfer, resulting in the conductivity change as the sensing signal. The excellent performance is mainly attributed to the heterojunction among MoS₂, MoO₂, and Fe₂Mo₃O₈ and the exceptional adsorption and catalytic activity of Fe₂Mo₃O₈/MoO₂@MoS₂‐900 °C for NH₃. This research presents a promising new material optimized for detecting NH₃ in exhaled breath and a new strategy for the early diagnosis and management of kidney disease.