Real‐time monitoring of respiration is vital for human health, especially for forecasting the sleep‐related diseases. A respiratory monitoring system with high accuracy, wearing comfort, portability, and environmental tolerance, is highly desirable, which however remains a big challenge. Here, a multimodal hydrogel sensor with excellent comprehensive performance is fabricated to monitor respiration and diagnose obstructive sleep apnea syndrome (OSAS). The synthetic cellulose‐based hydrogel exhibits good mechanical properties and extreme temperature tolerance, ascribing to the synergistic effects between chemical cross‐linking and multiple hydrogen bonding within the hydrogel network. The fabricated hydrogel sensor can independently monitor the mechanical variation and the thermal change via output signals of capacitance and resistance, respectively. These extraordinary properties of the hydrogel sensor enable the highly reliable and accurate monitoring of the respiratory events and diagnosis of OSAS. This work provides the new and practical way for real‐time respiratory monitoring and preventing the occurrence of sleep‐related diseases.