A sunlight-detecting sensor for vehicles is provided including a housing mountable on an inside of either a front window or a rear window. A sunlight sensor is provided on a top of the housing. The sunlight sensor includes a first sensor, a second sensor and a sensor holder. There is a printed circuit board (PCB) mounted between the sunlight sensor and the housing to transmit electrical signals that are proportional to an amount of sunlight measured on the sunlight sensor. There is cover made of transparent material and mounted on the top of the housing, and a blocker provided on the cover between the first and second sensors and inducing differences in amounts of sunlight measured by the first and second sensors when a position of sun is changed by a certain angle from a center of the sensors.