Cell–cell junction formation requires actin cytoskeletal remodeling. Here, we show that PLEKHG4B, a Rho-guanine nucleotide exchange factor (Rho-GEF), plays a crucial role in epithelial cell–cell junction formation. Knockdown of PLEKHG4B decreased Cdc42 activity and tended to increase RhoA activity in A549 cells. A549 monolayer cells showed ‘closed junctions’ with closely packed actin bundles along the cell–cell contacts, but PLEKHG4B knockdown suppressed closed junction formation, and PLEKHG4B-knockdown cells exhibited ‘open junctions’ with split actin bundles located away from the cell–cell boundary. In Ca2+-switch assays, PLEKHG4B knockdown delayed the conversion of open junctions to closed junctions and β-catenin accumulation at cell–cell junctions. Furthermore, PLEKHG4B knockdown abrogated the reduction in myosin activity normally seen in the later stage of junction formation. The aberrant myosin activation and impairments in closed junction formation in PLEKHG4B-knockdown cells were reverted by ROCK inhibition or LARG/PDZ-RhoGEF knockdown. These results suggest that PLEKHG4B enables actin remodeling during epithelial cell–cell junction maturation, probably by reducing myosin activity in the later stage of junction formation, through suppressing LARG/PDZ-RhoGEF and RhoA–ROCK pathway activities. We also showed that annexin A2 participates in PLEKHG4B localization to cell–cell junctions.This article has an associated First Person interview with the first author of the paper.