An optical hydrophone for measurement of the acoustic pressure distribution in a fluid medium has a light source that illuminates an area of a boundary region between an optically transparent body and the fluid medium. A light receiver measures the intensity of the light reflected from the illuminated area as a measurement of acoustic pressure, due to changes in the index of refraction of the fluid medium that are caused by the acoustic pressure. The illuminated area is smaller than the boundary surface formed between the optically transparent body and the fluid medium, thereby giving the hydrophone a longer service light and a high spatial resolution capacity.