A nonlinear optical fiber includes a core including a center core region, a core layer that is formed around the center core region and that has a refractive index lower than a refractive index of the center core layer, and at least one buffer core layer that is formed between the center core region and the core layer and that has a refractive index lower than the refractive index of the center core region and higher than the refractive index of the core layer; and a cladding that is formed around the core layer and that has a refractive index lower than the refractive index of the center core region and higher than the refractive index of the core layer. An effective core area at a wavelength of 1550 nm is equal to or smaller than 18 µm 2 . With this configuration, there is provided a nonlinear optical fiber having stable wavelength dispersion characteristics in the longitudinal direction even with a high optical nonlinearity so that a nonlinear optical phenomenon can be used with high efficiency, a nonlinear optical device employing the nonlinear optical fiber, and an optical signal processing apparatus employing the nonlinear optical fiber.