Background: Local drug delivery can increase antitumor exposure while limiting systemic toxicity, but chemotherapy-only local treatment may not fully control residual tumor growth in immunosuppressive tumor microenvironments. This study aimed to develop and preliminarily evaluate ffky-antiCD3, a CD3-recognition peptide-functionalized self-assembling peptide platform for intratumoral doxorubicin (DOX) delivery. Methods: The Nap aromatic group in a previous Nap-ffky scaffold was removed to improve aqueous dispersibility, and the CD3-recognition sequence AKMGEGGWGANDY was introduced to generate ffky-antiCD3. The peptide/formulation was characterized by reversed-phase high-performance liquid chromatography, mass spectrometry, TEM, circular dichroism spectroscopy, and a preliminary in vitro DOX release assay under tumor-mimicking acidic conditions. Antitumor efficacy, tumor histopathology, image-based CD3/CD8 semi-quantification, splenic IFN-γ levels, serum biochemistry, organ coefficients, and major-organ histology were assessed after repeated intratumoral treatment in B16-F10 melanoma-bearing C57BL/6 mice. Results: ffky-antiCD3 formed assemblies with a β-sheet-rich secondary structure. TEM observation further showed heterogeneous irregular/network-like supramolecular assemblies, and the preliminary release assay suggested slower apparent DOX release from ffky-antiCD3/DOX than from free DOX at pH 6.5. Among the tested groups, ffky-antiCD3/DOX produced the strongest short-term tumor-growth inhibition and the lowest endpoint tumor weight during the 10-day observation period. Ki67 staining decreased, and TUNEL signals increased after ffky-antiCD3/DOX treatment, supporting reduced proliferation and enhanced apoptosis-related damage. CD3/CD8 staining and exploratory splenic IFN-γ measurements indicated preliminary immune-related changes associated with ffky-antiCD3-containing formulations. Body weight, organ weights, serum biochemical markers, and major-organ H&E staining revealed no obvious short-term toxicity signals under the tested regimen. Conclusions: ffky-antiCD3/DOX represents a candidate local peptide-based chemo-immunomodulatory formulation. Its immune mechanism, release behavior, biodistribution, and long-term efficacy and safety require further validation before strong mechanistic or translational claims are made.