A leakage detection system configured to detect hydrocarbon leakage from an underwater pipeline in a body of water having a sensor configured to acquire signals related to the temperature of a liquid inside a chamber that is not watertight and which, in use, is full of water and extends along a portion of underwater pipeline susceptible to leakage, and a control unit, which is connected to the sensor and is configured to control if the temperature of the liquid in the chamber is within an expected range and emit a leakage signal when the temperature of the liquid in the chamber is outside the expected range.