Disclosed is an infrared optical sensor for the continuous analysis of a liquid flowing in a pipe. The sensor includes, in a housing, a central section of duct through which the liquid to be analyzed flows, and a spectral analysis device using a light beam, including an infrared source which emits a signal which is received by a receiving device having passed through the liquid, an optical component which allows the light beam to pass through the liquid, and a support plate which carries the infrared source and the receiving device. The liquid to be analyzed circulates through a loop formed by walls in the form of an arch of the optical component and by a projection of the housing in the optical component. A sealing gasket is compressed between the optical component and the housing, in order to prevent any diffusion of liquid on the interior of the housing.