Cells migrate by directing Rac1 and Cdc42 activities as well as actin polymerization towards their front. Previous studies proposed that this polarization process requires a local positive feed-back in the front involving Rac small GTPase and actin polymerization with PI3K likely playing a coor-dinating role. Here we show that PLEKHG3 is a PI3K-regulated RhoGEF for Rac1 and Cdc42 that se-lectively binds to newly polymerized actin in the front of migrating fibroblasts. Optogenetic inactiva-tion of PLEKHG3 showed that PLEKHG3 is indispensable not only for inducing but also for maintain-ing cell polarity. By selectively binding to newly polymerized actin, PLEKHG3 promotes local Rac1/ Cdc42 activation to induce more local actin polymerization which in turn promotes recruitment of more PLEKHG3 to induce and maintain frontness. Thus, autocatalytic reinforcement of PLEKHG3 localiza-tion to the front provides a molecular basis for the proposed positive feedback loop that is required for cell polarization and directed migration.