A device for holding a flap of a non-return valve in a closed position, which is configured to carry out hydraulic testing of a device or apparatus, includes a body which takes the form of a caliper with two arms that are articulated relative to each other, at a first of the two opposite ends thereof, about a first axis, in that it includes a holder articulated about the first axis and in that the two arms each has, at the second end thereof, a pad pivoting about axes that are parallel to the first axis.