Tumor-associated macrophages (TAMs) play an important role in tumor progression and metastasis. In this study, we developed a TAM-targeting poly lactic-co-glycolic acid (PLGA) nanoparticle (NP) that encapsulates a bisphosphonate drug, clodronate. Clodronate-loaded NPs killed macrophages more efficiently than the drug alone in vitro. The NPs functionalized with a TAM-targeting tissue-penetrating peptide, LyP-1, selectively accumulated in CD11b-positive (macrophage-rich) regions in mouse 4T1 breast tumor xenografts. Furthermore, clodronate-loaded LyP-1-functionalized NPs reduced the number of macrophages selectively in the 4T1 tumors, and modestly inhibited tumor growth. Our results support the idea that LyP-1-functionalized clodronate-loaded PLGA NPs could be a promising anti-tumor agent.