The invention relates in particular to a safety valve (1) for a fluid circuit under pressure (P), the said valve comprising a body (2), a piston (3) that can move relative to the body between a closed position and a purge position, a thermally deformable element (5) acting to cause the piston to move from the closed position to the purge position when a purge temperature is exceeded so as to connect the circuit to a discharge line (16), the valve further comprising an overpressure relief device sensitive to the pressure of the fluid and configured in such a way as to connect the circuit to the discharge line when the pressure (P) in the circuit reaches a trigger pressure.