It is intended to provide a novel contrast agent for photoacoustic imaging that is highly capable of binding to a target molecule and generates high photoacoustic signals. The present invention provides a contrast agent for photoacoustic imaging represented by Formula 1: MNP−((L)l−(P)m)n   (Formula 1) (wherein MNP represents a particle containing an iron oxide particle; L represents a linker molecule; P represents a ligand molecule; l represents 0 or 1; and m and n represent an integer of 1 or larger), the contrast agent for photoacoustic imaging including: a particle containing an iron oxide particle that absorbs light in a near-infrared region; and at least one or more ligand molecule(s) immobilized on the particle containing an iron oxide particle, wherein the immobilization density of the ligand molecule is equal to or higher than the cell surface density of a target molecule.