Compact and smart optical gas sensors have attracted significant attention over the past few decades. Among the materials used for developing such gas sensors, group-IV materials, including silicon, germanium, carbon allotropes, and their compounds, are considered the most promising candidates. By virtue of their inherent compatibility with the CMOS fabrication process in the mature microelectronics industry, on-chip optical gas sensors based on group-IV materials have merits of appreciable sensitivity and high-density integration. Moreover, such sensors have promising potential to be integrated with other electronic or photonic devices for on-chip signal processing and communication, which are expected to enable versatile applications in the internet of things, point-of-care testing, and information and communication technology. This paper is the review of basic principles, state-of-the-art devices, and cutting-edge applications of on-chip optical gas sensors based on group-IV materials and discussions of their prospects.