Cytokine receptors are randomly distributed on the cell surface membrane and are activated upon binding of their extracellular ligands to mediate downstream cellular activities. We hypothesized that pharmaceutical clustering of ligand-bound, activated receptors may lead to heretofore unrealized gain-of-function with therapeutically desirable properties. We here describe an engineered bifunctional cytokine borne of the fusion of Granulocyte Macrophage Colony Stimulating Factor (GMCSF) and Interleukin-9 (IL9) (hereafter GIFT9 fusokine) and demonstrate that it chaperones co-clustering of the functionally unrelated GMCSF receptor (GMCSFR) and IL9 receptor (IL9R) on cell surface of target cells. We demonstrate that GIFT9 treatment of MC/9 cells leads to transhyperphosphorylation of IL9R-associated STAT1 by GMCSFR-associated JAK2. We also show that IL9R-associated JAK1 and JAK3 augment phosphorylation of GMCSFR-linked STAT5. The functional relevance of these synergistic JAK/STAT transphosphorylation events translates to an increased mitogenic response by GMCSFR/IL9R-expressing primary marrow mast cells. The notion of inducing heterologous receptor clustering by engineered fusokines such as GIFT9 opens the door to a novel type of biopharmaceutical platform where designer fusokines modulate cell physiology through clustering of targeted receptor complexes.