Sensitivity and selectivity of polypyrrole (PPy) toward NH3, CO2, and CO have been studied at density functional theory (DFT). PPy oligomers are used both in the doped (PPy+) and neutral (PPy) form for their sensing abilities to realize the best state for gas sensing. DFT calculations are performed at the hybrid functional, B3LYP/6-31G­(d), level of theory. Detection/interaction of CO is investigated from carbon CO(1) and oxygen termini of CO CO(2). Interaction energies and charge transfer are simulated which reveal the sensing ability of PPy toward these gases. Furthermore, these results are supported by frontier molecular orbital energies and band gap calculations. PPy, in both the doped and neutral state, is more sensitive to NH3 compared to CO2 and CO. More interestingly, NH3 causes doping of PPy and dedoping of PPy+, providing evidence that PPy/PPy+ is an excellent sensor for NH3 gas. UV–vis and UV–vis–near-IR spectra of nPy, nPy+, and nPy/nPy+–X complexes demonstrate strong interaction of PPy/PPy+ with these atmospheric gases. The better response of PPy/PPy+ toward NH3 is also consistent with the experimental observations.