Polycyclic aromatic hydrocarbons (PAHs) are organic compounds composed of multiple aromatic rings resulting from oil spills and industrial pollution, many of which are capable of causing long-term health damage. These chemicals, a large class of uncharged non-polar organic compounds comprised of two or more condensed benzene rings arranged in various configurations, are partially soluble in water and are particularly harmful to aquatic ecosystems. Herein we demonstrate the first nanofibrous polymer-based on-site 1 colorimetric sensor for the detection of PAHs at environmentally-relevant concentrations. Electrospinning is used to create silica nanofibers, which are necessary as a reaction substrate and provide both hydrophobicity as well as support for the colorimetric reaction to occur. The volatile PAHs are concentrated in the vapor phase via a shift in the vapor-liquid equilibrium, allowing for a more sensitive detection range than in the liquid phase. Formaldehyde, under acidic conditions, is used to colorimetrically detect the PAHs—reacting to form brown quinoid compounds on top of the white silica fiber mat. Image analysis is further used to quantify the PAH concentration based off the intensity of the color change. This robust and portable sensor is an uncomplicated alternative to the traditional, expensive analytical equipment (HPLC, GC-MS), and allows for on-site and rapid water monitoring.