The biological effects of circularly polarized light on living cells are considered to be negligibly weak. Here, we show that the differentiation of neural stem cells into neurons can be accelerated by circularly polarized photons when DNA-bridged chiral assemblies of gold nanoparticles are entangled with the cells’ cytoskeletal fibres. By using cell-culture experiments and plasmonic-force calculations, we demonstrate that the nanoparticle assemblies exert a circularly-polarized-light-dependent force on the cytoskeleton, and that the light-induced periodic mechanical deformation of actin nanofibres with a frequency of 50 Hz stimulates the differentiation of neural stem cells into the neuronal phenotype. When implanted in the hippocampus of a mouse model of Alzheimer’s disease, neural stem cells illuminated following a polarity-optimized protocol reduced the formation of amyloid plaques by more than 70%. Our findings suggest that circularly polarized light can guide cellular development for biomedical use.