Carbon monoxide (CO) is an essential gaseous signaling molecule in the human body. Toward the controlled delivery of CO to the target tissues or cells, nanomaterial-based CO donors have attracted growing attention. Here, we present CO-releasing polymeric nanoparticles (CONPs) prepared by simple mixing of phenylboronic acid-containing framboidal nanoparticles with the catechol-bearing CO-donor Ru(CO)3Cl(L-DOPA) via phenylboronic acid-catechol complexation. The CONPs release CO in response to cysteine and suppress the production of the pro-inflammatory mediators interleukin 6 (IL-6) and nitric oxide (NO) in lipopolysaccharide (LPS)-stimulated murine macrophages. This CONP platform may show promise in therapeutic applications of CO.