Breast cancer remains a leading cause of morbidity and mortality among women worldwide, emphasizing the urgent need for enhanced diagnostic and therapeutic approaches. Leucine-rich-alpha-2-glycoprotein 1 (LRG1) has emerged as a notable target due to its markedly elevated expression in breast tumors, suggesting the viability of LRG1 as a theranostic target. In our study, we employed phage display technology to identify a peptide, termed ET, that binds to LRG1 with a dissociation constant of 48.4 μM. After modified with fluorescent cyanine dye, the ET peptide showcased effective tumor-targeting imaging across three different primary breast tumor models and a metastatic breast tumor model. We also undertook a comprehensive safety evaluation, which verified the good biosafety credentials of ET peptide. In summary, the ET peptide identified in this study shows effective LRG1-targeting ability both in vitro and in vivo, thus exhibiting immense potential for clinical translation.