[en] Highlights: • Penta-ZnO2 is dynamically and mechanically stable. • Penta-ZnO2 is much more flexible than graphene-like ZnO and penta-graphene. • The visible light transmittance of penta-ZnO2 is almost 100%. • Penta-ZnO2 is a promising candidate for transparent electrodes. Using first-principles calculations, we predict the existence of two dimensional (2D) pentagonal ZnO₂ (penta-ZnO₂) sheet and confirm that penta-ZnO₂ is dynamically and mechanically stable. Due to its unique atomic configuration, penta-ZnO₂ has intrinsic negative Possion's ratio, contrary to the most existing materials. Furthermore, Penta-ZnO₂ is much more flexible than graphene-like ZnO and penta-graphene in elasticity. Using the Heyd-Scuseria-Ernzerhof (HSE06) functional, the calculated indirect energy band gap is 4.53 eV, which is corresponding with optical gap (4.54 eV). More interesting, in the visible range of light, we find that penta-ZnO₂ has very low reflectance (0.0035%). Together with the expected zero absorption below the band gap, the light transmittance of penta-ZnO₂ is almost 100% in a broad spectral range. Our study demonstrates that penta-ZnO₂ possesses unique stable structure and promising mechanical, electronic and optical properties, which offers great potential in a wide range of applications in optoelectronics. Especially, the high flexibility and high light transmission make the penta-ZnO₂ to be a promising candidate for transparent electrodes.