Vesicle budding for Golgi-to-plasma membrane trafficking is a key step in secretion. Proteins that induce curvature of the Golgi membrane are predicted to be required, by analogy to vesicle budding from other membranes. Here, we demonstrate that GOLPH3, upon binding to the phosphoinositide PI4P, induces curvature of synthetic membranes in vitro and the Golgi in cells. Moreover, efficient Golgi-to-plasma membrane trafficking critically depends on the ability of GOLPH3 to curve the Golgi membrane. Interestingly, uncoupling of GOLPH3 from its binding partner MYO18A results in extensive curvature of Golgi membranes, producing dramatic tubulation of the Golgi, but does not support forward trafficking. Thus, forward trafficking from the Golgi to the plasma membrane requires the ability of GOLPH3 both to induce Golgi membrane curvature and to recruit MYO18A. These data provide fundamental insight into the mechanism of Golgi trafficking and into the function of the unique Golgi secretory oncoproteins GOLPH3 and MYO18A. Display omitted •GOLPH3 induces curvature of liposomes in vitro and the Golgi in cells•GOLPH3 induces curvature by insertion of a hydrophobic β-loop into the bilayer•Efficient Golgi-to-plasma membrane trafficking depends on GOLPH3-induced curvature•Golgi-to-plasma membrane trafficking also requires GOLPH3 interaction with MYO18A Rahajeng et al. show that GOLPH3, upon binding to PtdIns(4)P-containing lipid bilayers, induces membrane curvature. This membrane-shaping activity of GOLPH3 is required for efficient Golgi-to-plasma membrane trafficking but is not sufficient. GOLPH3 also must recruit MYO18A to the Golgi to enable efficient forward trafficking.