An isolated ultra-short γ-ray pulse is a unique tool for measuring ultrafast-physics processes, such as imaging intra-nuclear dynamics and inner-shell electron dynamics. Here, we propose an all-optical efficient scheme for generating isolated ultra-short γ-ray pulse from a laser-driven nanofoil. When a few-cycle circularly polarized laser pulse with an intensity of 1022 W cm−2 irradiates a nanofoil, the electrons in the nanofoil are pushed forwards collectively, forming a single relativistic electron sheet (RES) with a charge of nC. The electrons are substantially accelerated to high energies by the super-ponderomotive force of the laser. Then, a counter-propagating laser pulse with a peak intensity of 1021 W cm−2 collides with the RES, resulting in the generation of an isolated sub-femtosecond γ-ray pulse via nonlinear Compton scattering. The effect of laser polarization on the polarization degree of γ-rays is investigated by using a proof-of-principle calculation. It is shown that a highly polarized isolated γ-ray pulse with a cut-off energy of 100 MeV can eventually be generated in a head-on collision configuration when the scattering laser is linearly polarized. Such an isolated ultra-short polarized γ-ray source would provide critical applications in high-energy physics, laboratory astrophysics and nuclear physics.