Weak scattering and short optical interaction lengths have, until this work, prevented the observation of trace gas Raman spectra using photonic integrated circuitry. Raman spectroscopy is a powerful analytical tool, and its implementation using chip-scale waveguide devices represents a critical step toward trace gas detection and identification in small handheld systems. Here, we report the first Raman scattering measurements of trace gases using integrated nanophotonic waveguides. These measurements were made possible using highly evanescent rib waveguides functionalized with a thin cladding layer designed to reversibly sorb organophosphonates and other hazardous chemical species. Raman spectra were collected using 9.6 mm-long waveguides exposed to ambient trace concentrations of ethyl acetate, methyl salicylate, and dimethyl sulfoxide with one-sigma limits of detection in 100 s integration times equal to 600 ppm, 360 ppb, and 7.6 ppb, respectively. Our analysis shows that the functionalized waveguide Raman efficiency can be enhanced by over nine orders of magnitude compared to traditional micro-Raman spectroscopy, paving the way toward a sensitive, low-cost, miniature, spectroscopy-based trace gas sensor inherently suitable for foundry-level photonic integrated circuit manufacturing.