Plant pathogens secrete various cell wall-degrading enzymes that compromise host cell wall integrity and facilitate pathogen invasion. This study identified VdGH7a, a glycoside hydrolase family 7 (GH7) cellobiohydrolase from Verticillium dahliae, which demonstrated hydrolytic activity against 1,4-β-glucan. Notably, VdGH7a induced cell death in Nicotiana benthamiana when signal peptides were present, though this effect was inhibited by the carbohydrate-binding type-1 (CBM1) protein domain. The deletion of VdGH7a substantially reduced V. dahliae pathogenicity in cotton plants, as demonstrated by the mutants’ inability to penetrate cellophane membrane. These knockout mutants also exhibited reduced carbon source utilization capacity and increased sensitivity to osmotic and cell wall stresses. Through yeast two-hybrid screening, bi-molecular fluorescence complementation (BiFC), and luciferase complementation imaging (LCI), we identified that VdGH7a interacts with an osmotin-like protein (GhOLP1) in cotton. Virus-induced gene silencing of GhOLP1 resulted in decreased salicylic acid (SA) content and reduced resistance to V. dahliae in cotton, while heterologous overexpression of GhOLP1 in Arabidopsis enhanced both resistance and SA signaling pathway gene expression. These results reveal a virulence mechanism wherein the secreted protein VdGH7a from V. dahliae interacts with GhOLP1 to activate host immunity and contribute significantly to plant resistance against V. dahliae.