An ab initio density functional theory study on the candidacy of single-walled black phosphorus nanotubes (BPNTs) towards sensing several common toxic gas molecules (NH3, CO, NO, NO2, and SO2) was conducted. Various adsorption characteristics, including the geometry, adsorption energy, charge transfer, band structure, and curvature effect were examined. Compared with MLBP, BPNTs are found to generally exhibit similar adsorption energy towards these molecules, whereas show selectively much stronger interaction with NO2. Analysis of charge density difference and band structure also indicates the electronic properties of BPNTs are significantly altered after the adsorption of NO2: transferring an indirect band gap of ~0.3 eV for pristine (0, 9) BPNT to a metallic system. These facts collectively indicate the higher capability, sensitivity, and selectivity of BPNTs in the detection of NO2 compared to its planar counterpart. Moreover, the NO2 adsorption is found to be influenced by the curvature of BPNTs. Overall, findings from the present study indicate that BPNTs may serve as potential building blocks for high-performance gas sensors towards NO2 sensing.