H2S gas in oral exhalation is a critical biomarker for early periodontitis diagnosis, yet practical on-site detection via portable sensors remains challenging. Herein, we report a room-temperature H2S sensor based on CAU-17-derived Yb-doped Bi2S3 (Yb-Bi2S3) nanoribbons for rapid periodontitis screening. The sensor exhibits segmented linearity ranging from 5 ppb to 5 ppm, with a 3.31 response at 100 ppb, 11 s response time, and superior selectivity. Mechanism analysis combined with density functional theory calculations demonstrates that the superior detection performance originates from Yb doping-induced lattice distortion in Bi2S3, which enhances oxygen anion formation and specific H2S adsorption capability. Notably, the sensor quantifies H2S production from cultured Porphyromonas gingivalis, corroborating the correlation between oral pathogens, H2S, and periodontitis. Additionally, it effectively discriminates exhaled H2S levels on-site between healthy individuals and periodontitis patients in clinical validation. This work provides a promising platform for sensitive biomarker detection, paving the way for advanced intelligent medical diagnostics.