The GTPase RhoA promotes contractile ring assembly and furrow ingression during cytokinesis. Although many factors that regulate RhoA during cytokinesis have been characterized, the spatiotemporal regulatory logic remains undefined. We have developed an optogenetic probe to gain tight spatial and temporal control of RhoA activity in mammalian cells and demonstrate that cytokinetic furrowing is primarily regulated at the level of RhoA activation. Light-mediated recruitment of a RhoGEF domain to the plasma membrane leads to rapid induction of RhoA activity, leading to assembly of cytokinetic furrows that partially ingress. Furthermore, furrow formation in response to RhoA activation is not temporally or spatially restricted. RhoA activation is sufficient to generate furrows at both the cell equator and cell poles, in both metaphase and anaphase. Remarkably, furrow formation can be initiated in rounded interphase cells, but not adherent cells. These results indicate that RhoA activation is sufficient to induce assembly of functional contractile rings and that cell rounding facilitates furrow formation.