SPIN90/WISH (SH3 protein interacting with Nck, 90 kDa/Wiskott-Aldrich syndrome protein (WASP) interacting SH3 protein) regulates actin polymerization through its interaction with various actin-regulating proteins. It is highly expressed in the brain, but its role in the nervous system is largely unknown. We report that it is expressed in dendritic spines where it associates with PSD-95. Its overexpression increased the number and length of dendritic filopodia/spines via an N-WASP-independent mechanism, and knock down of its expression with small interfering RNA reduced dendritic spine density. The increase in spinogenesis is accompanied by an increase in synaptogenesis in contacting presynaptic neurons. Interestingly, PSD-95-induced dendritic spinogenesis was completely abolished by knock down of SPIN90/WISH. Finally, in response to chemically induced long-term potentiation, SPIN90/WISH associated with PSD-95 and was redistributed to dendritic spines. Our results suggest that SPIN90/WISH associates with PSD-95, and so becomes localized to dendritic spines where it modulates actin dynamics to control dendritic spinogenesis. They also raise the possibility that SPIN90/WISH is a downstream effector of PSD-95-dependent synaptic remodeling.