To detect macrophages in atherosclerotic plaques, plasmonic gold nanoparticles are introduced as a contrast agent for intravascular photoacoustic imaging. The phantom and ex vivo tissue studies show that the individual spherical nanoparticles, resonant at 530 nm wavelength, produce a weak photoacoustic signal at 680 nm wavelength while photoacoustic signal from nanoparticles internalized by macrophages is very strong due to the plasmon resonance coupling effect. These results suggest that intravascular photoacoustic imaging can assess the macrophage-mediated aggregation of nanoparticles and therefore identify the presence and the location of nanoparticles associated with macrophage-rich atherosclerotic plaques.