Hexagonal boron nitride (hBN) hosts long-lived phonon polaritons, yielding a strong mid-infrared (mid-IR) electric field enhancement and concentration on the nanometer scale. It is thus a promising material for highly sensitive mid-IR sensing and spectroscopy. In addition, hBN possesses high chemical and thermal stability as well as mechanical durability, making it suitable for operation in demanding environments. In this work, we demonstrate a mid-IR CO2 gas sensor exploiting phonon polariton (PhP) modes in hBN nanoresonators functionalized by a thin CO2-adsorbing polyethylenimine (PEI) layer. We find that the PhP resonance shifts to lower frequency, weakens, and broadens for increasing CO2 concentrations, which are related to the change of the permittivity of PEI upon CO2 adsorption. Moreover, the PhP resonance exhibits a high signal-to-noise ratio even for small ribbon arrays of 30 × 30 μm2. Our results show the potential of hBN nanoresonators to become a novel platform for miniaturized phonon-enhanced SEIRA gas sensors.