In this paper, a new multilayer integrated optical sensor (MIOS) for ammonia detection at room temperature is proposed and characterized. The sensor is integrated on a single-mode TE0-TM0 planar polymer waveguide and based on polyaniline (PANI) sensitive material. A polymethyl methacrylate (PMMA) passive layer is deposited between the waveguide core and PANI sensitive layer in order to decrease optical losses induced by evanescent wave/sensitive material coupling. The design of this new sensor is discussed. Moreover, in order to investigate the feasibility of this sensor, the sensing properties to ammonia at room temperature are studied. A significant change is observed in the guided light output power after the sensor is exposed to ammonia gas, due to PANI absorption coefficient variation. This new ammonia sensor shows fast response and recovery times, good reversibility and repeatability. The metrological parameters (sensitivity, response time and recovery time) of the sensor are strongly influenced by the interaction length (length of sensing region) and the PANI forms (doped and dedoped). The sensor has a logarithmic linear optical response within the ammonia concentration range between 92 to 4618 ppm. These experimental results demonstrate that the MIOS structure presents a potential innovation to elaborate integrated optical sensor based on non transparent (opaque) sensitive material.