A sensor for a fluid duct in a fluid housing has a sensor housing, which has an insert part which complements a receiving part of the fluid housing, and a sealing element, which is arranged on a sealing section of the insert part and is configured to seal off an intermediate space between the sealing section and the receiving part when the insert part is inserted in an insertion direction into the receiving part in a final assembly position. The insert part has a securing section with a latching groove which is configured for a spring-elastic securing element to be able to engage into the latching groove in the final assembly position in order to fasten the sensor to the fluid housing. A sensor assembly having a sensor of this kind is also provided.