Activation of murine glucocorticoid-induced tumor necrosis factor-related receptor (mGITR) by its natural ligand (GITRL) or antiGITR agonist mAb enhances T-cell responses, inhibits regulatory T-cell (Treg)-mediated suppression and induces tumor immunity in a variety of murine tumor models. However, systemic administration of these costimulatory agents can lead to global T-cell activation and autoimmunity. To specifically manipulate the T-cell compartment in the tumor microenvironment we propose to target the tumor infiltrating T cells with a bispecific mGITRL fusion protein. For that purpose, mGITRL is linked to a single-chain antibody targeting fibroblast activation protein (FAP) as FAP expression is restricted to cancer-associated fibroblasts (CAFs) found in the stroma of epithelial cancers. AntiFAP-mGITRL fusion protein forms dimers and binds to murine GITR with 1.2 μM affinity and to murine FAP with 4.5 nM. The construct is able to costimulate CD8+ and CD4+ effector T cells resulting in increased proliferation, IFN-γ and IL-2 production. This costimulatory effect is enhanced when the fusion protein is bound to a FAP-positive cell line mimicking FAP CAFs. In suppression assays, membrane-bound antiFAP-mGITRL is 100-fold more effective in overcoming Treg-mediated suppression than unbound fusion protein. These studies suggest that targeted tumor therapy with antiFAP-mGITRL fusion protein could induce tumor rejection while minimizing autoimmune side effects.