Targeted therapy is a revolutionary approach in cancer treatment, enabling the precise elimination of malignant cells with fewer side effects than conventional chemotherapy. Among clinically approved strategies, antibody-drug conjugates (ADCs) and immunotoxins represent a rapidly expanding class of biotherapeutics, often serving as theranostic tools when linked to diagnostic components. However, their production requires complex chemical conjugation, making it a technically and economically demanding process. Similarly, nanoparticle-based systems offer targeted delivery but usually rely on multi-step, poorly reproducible syntheses. Here, we introduce a novel type of genetically encoded self-assembling protein nanoparticles with targeting functionality. These nanometer-sized particles, which we propose to term "avisomes", self-assemble upon refolding of a fusion protein comprising the HER2-specific affibody Z and streptavidin (Z-Strp), combining the strengths of bioconjugates and nanoparticles while avoiding the drawbacks of chemical synthesis. Importantly, they are easily and reproducibly synthesized in E. coli. HER2 recognition was confirmed by flow cytometry and fluorescence microscopy. To confer cytotoxic activity, avisomes were loaded with doxorubicin (DOX), yielding Z-Strp-DOX nanoparticles that selectively killed HER2-overexpressing cells in vitro. In vivo, treatment with Z-Strp-DOX significantly suppressed the growth of HER2-positive tumors in BALB/c mice. Notably, therapy was well tolerated, with no abnormal hematological or biochemical changes, and it alleviated DOX-induced neutropenia. These findings establish avisomes as a highly promising theranostic platform for HER2-positive breast cancer. Its genetically encoded, self-assembling nature eliminates the major limitations of ADCs and synthetic nanoparticles, providing a scalable, reproducible, and cost-efficient alternative for next-generation targeted cancer therapy.