A flexible joint for conveying a fluid in a fluid pipe in order to unite two pipes in leaktight and flexible manner comprises a tubular vessel having an upstream end for flexible and leaktight connection with an upstream tube, a downstream end for leaktight connection with a downstream tube, and a tubular bellows fastened in leaktight manner to the inside of the vessel, thereby defining two concentric spaces that are mutually separated in leaktight manner in normal operation. An inner space is provided in which the fluid flows between the upstream end and the downstream end. A peripheral space is situated between the inner space and a side wall of the vessel and is filled with an incompressible liquid. The joint further includes at least one sensor for detecting a hydrocarbon fluid in the incompressible liquid if the seal is broken between the inner space and the peripheral space.