A testing or filling adapter (1) for a container to be filled with fluid, composed of a body (10), an attachment module (3), and a coupling system (30) having a coupled position, in which a sealed connection is established between the body and the attachment module, and an uncoupled position. The body includes a feed module (2) that comprises a plurality of conduits that include at least one fluid conduit (221) and is equipped with a safety valve (5) that can be moved between an open configuration and a closed configuration and is connected to an element mirror (51) having a first configuration in which the safety valve is in the open configuration and a second configuration in which the safety valve is in the closed configuration. The filling adapter further comprises a safety device (6) that cooperates with the coupling system, with it being possible to move said safety device between a locked position, in which the safety device locks the coupling system in the coupled position, and an unlocked position, in which the safety device enables the coupling system to be moved into the uncoupled position. The safety device cooperates with the mirror element so as to enable the mirror element to be in its first configuration when the safety device is in the locked position, and to force the mirror element to be in its second configuration when the safety device is moved from its locked position to its unlocked position.