Epithelial tube formation requires Rho-dependent actomyosin contractility that generates cellular forces to drive cell shape changes and rearrangement. Rho signaling is activated by G protein-coupled receptor (GPCR) signaling at the cell surface in both invertebrates and vertebrates. During Drosophila embryonic salivary gland (SG) invagination, the GPCR ligand Folded gastrulation (Fog) activates Rho signaling to drive apical constriction. Two GPCRs, Smog (ubiquitous) and Mist (mesoderm-specific), regulate myosin contractility downstream of Fog in the early Drosophila embryo. However, the SG receptor for Fog that translates Fog signal to cytoskeletal reorganization has not yet been identified. Using genetic suppression assay and in vitro cell contraction assay, we revealed that Smog transduces Fog signal to regulate Rho kinase (Rok) and myosin accumulation in the apicomedial region of SG cells to control apical constriction during invagination. We also discovered Fog-independent roles of Smog in maintaining epithelial integrity. smog loss results in reduced/mislocalized junctional myosin, which correlates with reduced and discontinuous signals of the apical determinant protein Crb and the key adherens junction protein E-Cad; this leads to a distorted embryonic morphology with an abnormally elongated SG placode. Moreover, smog null mutants also show enhanced bleb formation in the apical domain of SG cells during invagination, suggesting disorganized cortical actin networks upon smog loss. Genetic interaction tests with actin polymerizing/depolymerizing factors reveal that Smog regulates the cortical actin networks by regulating actin assembly/disassembly. Our study supports a model that Smog regulates apicomedial and junctional myosin pools in a Fog-dependent and -independent manner, respectively, and reveals a new role of Smog in regulating cortical actin organization during epithelial tube formation.