The study introduces a genetically encoded, photoactivatable Rac1 (PA-Rac1) that allows precise control of protein activity in living cells. PA-Rac1 is created by fusing the photoreactive LOV domain from phototropin to the N-terminus of Rac1, which sterically blocks Rac1 interactions until light activation unwinds a helix linking LOV to Rac1. This system can be reversibly and repeatedly activated using 458 or 473 nm light, generating localized cell protrusions and ruffles. Localized Rac activation or inactivation sufficient to produce cell motility and control direction of movement was demonstrated. Myosin was found to be involved in Rac-induced directionality but not in protrusion, while PAK was required for protrusion. PA-Rac1 was also used to elucidate the regulation of RhoA by Rac in cell motility, showing that localized Rac activation inhibits RhoA activity. Structural studies revealed that the LOV domain occludes effector binding in the dark state, and mutations at the Rac-LOV interface can be engineered to cage other GTPases. This work demonstrates the potential of PA-Rac1 as a versatile tool for controlling protein activities in living cells.The study introduces a genetically encoded, photoactivatable Rac1 (PA-Rac1) that allows precise control of protein activity in living cells. PA-Rac1 is created by fusing the photoreactive LOV domain from phototropin to the N-terminus of Rac1, which sterically blocks Rac1 interactions until light activation unwinds a helix linking LOV to Rac1. This system can be reversibly and repeatedly activated using 458 or 473 nm light, generating localized cell protrusions and ruffles. Localized Rac activation or inactivation sufficient to produce cell motility and control direction of movement was demonstrated. Myosin was found to be involved in Rac-induced directionality but not in protrusion, while PAK was required for protrusion. PA-Rac1 was also used to elucidate the regulation of RhoA by Rac in cell motility, showing that localized Rac activation inhibits RhoA activity. Structural studies revealed that the LOV domain occludes effector binding in the dark state, and mutations at the Rac-LOV interface can be engineered to cage other GTPases. This work demonstrates the potential of PA-Rac1 as a versatile tool for controlling protein activities in living cells.