An all-solid-state mixed potential type H₂S gas sensor was fabricated with yttria-stabilized zirconia (YSZ) solid electrolyte and Cobaltous stannate (Co₂SnO₄) sensing electrode (SE) for monitoring halitosis disease. The pure phase cubic spinel structure of Co₂SnO₄prepared by simple hydrothermal method and annealed at 1000 °C was confirmed based on X-ray diffraction (XRD), Raman, Field-emission scanning electron microscopy (FESEM) and Transmission electron microscope (TEM) technologies. The developed sensor showed the lowest detection limit of 100 ppb, fast response/recovery time of 44/97 s and high sensitivity of -54.4 mV/decade to 0.1−10 ppm H₂S. The sensing device also gave good reproducibility, selectivity, humidity resistance and long-term stability. Finally, the simulated test for diagnosing halitosis was conducted and the evident difference of sensing response of the present sensor between healthy breath (collecting from healthy volunteers) and halitosis breath (healthy breath blended 1 ppm H₂S) indicated that the fabricated gas sensor using Co₂SnO₄-SE showed great potential application value in halitosis monitoring. The present work has opened up a new window to noninvasive and online diagnostics of halitosis in medical diagnosis and treatment.