Backpack comprising: - a case defining a containing space for carrying a load and bounded at least by a fixed backrest, available in use in contact with the user's back, and by a front, opposite to the fixed backrest; - a movable backrest displaceable along said fixed backrest; - at least one shoulder strap for carrying the backpack on the shoulders, having a first end portion connected to said movable backrest and a second opposite end portion fastened to said fixed backrest or to one side of said case; - adjustment means for adjusting the position of said movable backrest and of said shoulder strap along a vertical axis of said fixed backrest; and - releasable locking means to fix said movable backrest and said shoulder strap in the desired position along said fixed backrest; wherein said adjustment means comprise an adjustment belt mounted on said fixed backrest in a movable way along said axis and carrying said movable backrest; characterized in that said fixed backrest comprises a pocket, delimited towards the outside by a flexible rear wall, and a stiffening panel housed in said pocket and shaped to adapt to the normal curvature of the human back; in that said rear wall and said stiffening panel comprise respective upper openings (29, 30) and respective lower openings (31, 32) aligned along said axis; and in that said adjustment belt is arranged passing through said upper openings (29, 30) and said lower openings (31, 32) and is wound to form a loop around said rear wall and said stiffening panel so as to be able to slide annularly and allow the adjustment of the position of said movable backrest and of said shoulder strap along said axis. - a case defining a containing space for carrying a load and bounded at least by a fixed backrest, available in use in contact with the user's back, and by a front, opposite to the fixed backrest; - a movable backrest displaceable along said fixed backrest; - at least one shoulder strap for carrying the backpack on the shoulders, having a first end portion connected to said movable backrest and a second opposite end portion fastened to said fixed backrest or to one side of said case; - adjustment means for adjusting the position of said movable backrest and of said shoulder strap along a vertical axis of said fixed backrest; and - releasable locking means to fix said movable backrest and said shoulder strap in the desired position along said fixed backrest; wherein said adjustment means comprise an adjustment belt mounted on said fixed backrest in a movable way along said axis and carrying said movable backrest; characterized in that said fixed backrest comprises a pocket, delimited towards the outside by a flexible rear wall, and a stiffening panel housed in said pocket and shaped to adapt to the normal curvature of the human back; in that said rear wall and said stiffening panel comprise respective upper openings (29, 30) and respective lower openings (31, 32) aligned along said axis; and in that said adjustment belt is arranged passing through said upper openings (29, 30) and said lower openings (31, 32) and is wound to form a loop around said rear wall and said stiffening panel so as to be able to slide annularly and allow the adjustment of the position of said movable backrest and of said shoulder strap along said axis.