Cyclin E regulates the cell cycle transition from G1 to S phase and is degraded before entry into G2 phase. Here we show that RhoBTB3, a Golgi-associated, Rho-related ATPase, regulates the S/G2 transition of the cell cycle by targeting cyclin E for ubiquitylation. Depletion of RhoBTB3 arrested cells in S phase, triggered Golgi fragmentation, and elevated cyclin E levels. On the Golgi, RhoBTB3 bound cyclin E as part of a Cullin3 (CUL3)-dependent RING-E3 ubiquitin ligase complex comprised of RhoBTB3, CUL3, and RBX1. Golgi association of this complex was required for its ability to catalyze cyclin E ubiquitylation and allow normal cell cycle progression. These experiments reveal a novel role for a Ras superfamily member in catalyzing cyclin E turnover during S phase, as well as an unexpected, essential role for the Golgi as a ubiquitylation platform for cell cycle control.