A fastening clamp for fixing a terminal block in a wall opening in a housing wall, having a clamp housing and at least one clamping element. The fastening clamp allows the terminal block to be mounted in and removed from a wall opening in a housing wall with a small width in that the clamping element has a resilient snap arm, having a foot region fastened to the clamp housing and a free end forming a clamping face, the snap arm extending along the side face of the clamp housing so that the clamping face acts on an edge of the wall opening when the fastening clamp is in the mounted state. A blocking element is displaceably mounted on the clamp housing for movement between a first position in which the resilient snap arm can be deflected, and a second position in which the blocking element cannot be deflected.