Triple-negative breast cancer (TNBC) presents significant challenges due to its aggressive behavior and lack of targeted treatments. High-resolution imaging techniques and targeted nanoparticles offer potential solutions for early detection and monitoring of TNBC. In this study, we developed and characterized solid lipid nanoparticles (SLNs) conjugated with a C-peptide derived from endostatin to target integrin αvβ, overexpressed in TNBC. These SLNs were loaded with superparamagnetic iron oxide nanoparticles (SPIONs) for enhanced magnetic resonance imaging (MRI) and radiolabeled with technetium-99m (Tc) for single-photon emission computed tomography (SPECT), enabling dual-modality imaging. Extensive characterization of the nanoparticles was performed utilizing a variety of advanced techniques, including dynamic light scattering (DLS), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD), vibrating sample magnetometry (VSM), field-emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM). This comprehensive analysis validated the successful synthesis and functionalization of the nanoparticles, along with their remarkable magnetic properties, while also revealing their distinct spherical morphology, optimal size, uniform distribution, and colloidal stability. The conjugation of C-peptide significantly enhanced the targeting efficiency in vitro, as evidenced by the MTT and receptor-binding assays in 4T1 cells using flow cytometry and MRI. In vivo studies using a 4T1 murine model demonstrated that peptide-conjugated SLNs accumulated in tumor tissues, providing superior contrast in MRI and enhanced tumor-specific localization in SPECT imaging. Biodistribution analysis confirmed reduced off-target accumulation, particularly in the liver, compared to nontargeted formulations. Collectively, C-peptide-conjugated SLNs provide a promising dual-modality imaging platform for TNBC, offering improved diagnostic accuracy and tumor targeting.