Breast cancer remains the most prevalent malignancy affecting women worldwide. Nanomedicine offers promising strategies to drastically enhance the efficacy of current treatments by enabling spatiotemporal control over cancer diagnostics and therapy. Here, we developed a triple-modal nanocarrier integrating chemo-, photothermal and photodynamic therapies within a single nanoplatform to address the limitations of conventional monotherapies. The nanoparticles were synthesized via polyelectrolyte complexation of sodium alginate and oligochitosan, yielding a biocompatible, non-toxic nanosystem suitable for localized therapies. The designed nanocarriers displayed an average hydrodynamic diameter of 206.1 ± 6.8 nm, with a ζ-potential of −23.8 ± 2.5 mV, features that favor their tumor retention. Additionally, they demonstrated effective encapsulation of doxorubicin and indocyanine green, high photostability, and protection of the loaded drugs from degradation, while remaining responsive to near-infrared stimulation. In vitro , the nanoparticles demonstrate low drug release coupled with substantial cellular uptake and pronounced cytotoxicity against triple-negative breast cancer cells. These results highlight the potential of the designed alginate–oligochitosan-based nanoplatform as a multimodal therapeutic system, capable of additively combining multiple treatment modalities for localized breast cancer therapy, with predicted improvements in therapeutic efficacy and reduced off-target effects, pending validation in further studies.