<p>This work reports an on-chip integrated non-dispersive infrared CO₂ sensor, in which a microelectromechanical system infrared emitter and a thermopile detector are monolithically integrated on a silicon substrate. Portable CO₂ monitoring, including wearable respiratory monitoring and portable environmental sensing, imposes stringent requirements on sensor miniaturization and rapid response. Conventional non-dispersive infrared sensors, constrained by discrete optical chambers and board-level packaging, face inherent trade-offs among compact size, fast response, detection range, and stability. To address these challenges, the optical chamber and device layout are co-optimized based on radiative transfer and thermal coupling principles to enhance infrared utilization while suppressing thermal crosstalk. Simulations show that the compact chamber achieves an effective optical path length of 11 mm with an optical coupling efficiency of 91.8%. An orthogonal optical–gas path configuration is further adopted to shorten the gas diffusion distance. The packaged sensor module remains highly compact, with dimensions smaller than 21 mm × 10 mm × 5 mm. Under a driving voltage of 2.5 V and a modulation frequency of 5 Hz, the sensor exhibits a response time of ~3 s, power consumption of 31 mW at a duty cycle of 10%, and a detection limit of 146 ppm, while maintaining stable performance over a wide temperature and humidity range. The on-chip integrated approach enables significant footprint reduction without compromising sensing performance, providing a promising pathway for portable CO₂ monitoring.</p><p></p>