An impact-assisted knife, for example for opening shellfish, which knife includes a blade associated with a gripping handle, which blade includes a front part and a rear part, and which gripping handle includes an internal cavity of longitudinal axis, a front end located on the side of said blade and a rear end, said blade being provided mobile in translation with respect to said gripping handle, at the front end of the latter and on said longitudinal axis, between two limit positions, a so-called "extracted position" and a so-called "retracted position", and said gripping handle including an impact mechanism, integrated in its internal cavity, which impact mechanism is adapted, when said blade is in the retracted or at least partially retracted position, to apply an impact against said rear part of said blade, in the direction of said longitudinal axis, so as to tend to move the latter to its extracted position, which impact mechanism includes: - a trigger member mounted mobile in translation in said internal cavity, on said longitudinal axis, at the front end of said gripping handle, which trigger member includes a front face arranged so as to form a head for hitting against said rear part of said blade, and a rear end in the form of a rod of longitudinal axis, provided with a free end, which trigger member is associated with elastic return means arranged so as to tend to push it towards said blade et to place the latter in the extracted position, - a hammer mounted mobile in translation in said internal cavity, on said longitudinal axis, at the rear end of said gripping handle, which hammer includes a front face through which is open a blind bore, of longitudinal axis, and provided with a bottom, which hammer is associated with elastic means arranged so as to tend to push it towards the front end of said gripping handle, against a front position stop limiting the progression thereof forward, said rod of said trigger member being adapted to be receivable in said blind bore of said hammer, characterized in that the free end of said rod of the trigger member is arranged so as to come in rest against the front face of said hammer, by offsetting, to the perimeter of said blind bore, so as to allow the compression of said elastic means associated with said hammer, when a force is applied on the front part of said blade, to bring the latter in its retracted position, and so as to allow the storage of energy in said elastic means, and in that said impact mechanism also includes means (52, 31, 26) for providing the coaxiality between the axis of the rod of said trigger member and the axis of the blind bore of said hammer, after compression, at least partially, of said elastic means of said hammer, the introduction of said rod into said blind bore, under the effect of said coaxiality means (52, 31, 36), causing the restitution to said hammer of the energy stored in said elastic means, said hammer then forming a piston and moving towards said trigger member and said blade, up to the contact by impact between said bore bottom of the hammer and the end of said rod of said trigger member, said contact by impact being transmitted to said blade, the size of said rod of the trigger member and that of said blind bore of the hammer being adapted so that said contact by impact occurs before the bearing of said hammer against its front position stop. - a trigger member mounted mobile in translation in said internal cavity, on said longitudinal axis, at the front end of said gripping handle, which trigger member includes a front face arranged so as to form a head for hitting against said rear part of said blade, and a rear end in the form of a rod of longitudinal axis, provided with a free end, which trigger member is associated with elastic return means arranged so as to tend to push it towards said blade et to place the latter in the extracted position, - a hammer mounted mobile in translation in said internal cavity, on said longitudinal axis, at the rear end of said gripping handle, which hammer includes a front face through which is open a blind bore, of longitudinal axis, and provided with a bottom, which hammer is associated with elastic means arranged so as to tend to push it towards the front end of said gripping handle, against a front position stop limiting the progression thereof forward,