Abstract Immunotherapeutic strategies such as T cell-recruiting bispecific antibodies (bsAb), chimeric antigen receptor (CAR) T cells, and immune checkpoint inhibition have revolutionized oncological treatment. However, many patients do not respond to treatment with the so far available therapeutics, others for limited time only. Various tumor immune evasion mechanisms have been reported to counteract efficiency of T cell engaging therapeutics. Thrombocytes have recently been described to affect cancer biology by mediating epithelial-mesenchymal transition, tumor invasion, metastasis and notably also immune evasion. Here we report that, upon treatment of prostate cancer patients within a clinical trial evaluating a novel PSMAxCD3 bsAb (NCT04104607), we observed profound treatment-associated platelet activation, mirrored by a decrease of total platelet count. Upon modelling the treatment setting, we found that platelet activation significantly reduced bsAb-mediated CD4+ and CD8+ T cell reactivity as revealed by impaired T cell activation, reduced secretion of perforin and ultimately inhibition of target cell lysis. The immunosuppressive effect mediated by platelets occurred in a TGFβ-dependent manner and was not restricted to the PSMAxCD3 bsAb, but was also observed with various other CD3-directed bispecific constructs including the clinically approved CD19xCD13 bsAb Blinatumomab. BsAb-mediated T cell reactivity could generally be restored by inhibiting platelets using dabigatran, but also specifically by blocking the TGFβ axis. Together, our findings unravel that platelets undermine the efficacy of T cell-recruiting bsAb and identify modulation of platelet function and TGFβ as means to reinforce the effectiveness of bsAb treatment. Citation Format: Martina S. Lutz, Boris Klimovich, Stefanie Maurer, Jonas S. Heitmann, Melanie Maerklin, Latifa Zekri, Gundram Jung, Helmut R. Salih, Clemens Hinterleitner. Platelet-derived TGFβ undermines treatment efficacy of t cell recruiting bispecific antibodies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2859.