Glucagon hypersecretion from pancreatic islet a-cells exacerbates hyperglycemia in type 1 diabetes (T1D) and type 2 diabetes. Still, the underlying mechanistic pathways that regulate glucagon secretion remain controversial. Among the three complementary main mechanisms (intrinsic, paracrine, and juxtacrine) proposed to regulate glucagon release from a-cells, juxtacrine interactions are the least studied. It is known that tonic stimulation of a-cell EphA receptors by ephrin-A ligands (EphA forward signaling) inhibits glucagon secretion in mouse and human islets and restores glucose inhibition of glucagon secretion in sorted mouse a-cells, and these effects correlate with increased F-actin density. Here, we elucidate the downstream target of EphA signaling in a-cells. We demonstrate that RhoA, a Rho family GTPase, plays a key role in this pathway. Pharmacological inhibition of RhoA disrupts glucose inhibition of glucagon secretion in islets and decreases cortical F-actin density in dispersed a-cells and a-cells in intact islets. Quantitative FRET biosensor imaging shows that increased RhoA activity follows directly from EphA stimulation. We show that in addition to modulating F-actin density, EphA forward signaling and RhoA activity affect a-cell Ca2+ activity in a novel mechanistic pathway. Finally, we show that stimulating EphA forward signaling restores glucose inhibition of glucagon secretion from human T1D donor islets.