System for monitoring the weight adjustment of a dumbbell, which can be used by a user to perform a gymnastic exercise, comprising: a dumbbell, provided with a handle, having an axial development along an axis, able to rotate around said axis, clockwise and counterclockwise, and provided with a first end and a second end, a first plurality of weights, wherein each weight can be individually coupled to said first end, when said handle rotates in one direction, and decoupled from said first end, when said handle rotates in the opposite direction, a second plurality of weights, wherein each weight can be individually coupled to said second end, when said handle rotates in one direction, and decoupled from said second end, when said handle rotates in the opposite direction, wherein said system comprises an element provided with a plurality of reflective portions (321 a,b,...,n), at least one detection device able to detect data coming from at least one reflective portion (321 k) and able to send said data, and a logic control unit able to receive said data sent by said at least one detection device and to associate to said data a weight of said first and second plurality of weights, characterised in that said detection device comprises at least one sensor, able to emit electromagnetic waves and receive electromagnetic waves reflected from at least one portion of said plurality of reflective portions (321 a,b,...,n) wherein said at least one sensor is arranged in said first end and/or in said second end of said handle. a dumbbell, provided with a handle, having an axial development along an axis, able to rotate around said axis, clockwise and counterclockwise, and provided with a first end and a second end, a first plurality of weights, wherein each weight can be individually coupled to said first end, when said handle rotates in one direction, and decoupled from said first end, when said handle rotates in the opposite direction, a second plurality of weights, wherein each weight can be individually coupled to said second end, when said handle rotates in one direction, and decoupled from said second end, when said handle rotates in the opposite direction, a handle, having an axial development along an axis, able to rotate around said axis, clockwise and counterclockwise, and provided with a first end and a second end, a first plurality of weights, wherein each weight can be individually coupled to said first end, when said handle rotates in one direction, and decoupled from said first end, when said handle rotates in the opposite direction, a second plurality of weights, wherein each weight can be individually coupled to said second end, when said handle rotates in one direction, and decoupled from said second end, when said handle rotates in the opposite direction, wherein said system comprises an element provided with a plurality of reflective portions (321 a,b,...,n), at least one detection device able to detect data coming from at least one reflective portion (321 k) and able to send said data, and a logic control unit able to receive said data sent by said at least one detection device and to associate to said data a weight of said first and second plurality of weights, characterised in that said detection device comprises at least one sensor, able to emit electromagnetic waves and receive electromagnetic waves reflected from at least one portion of said plurality of reflective portions (321 a,b,...,n) wherein said at least one sensor is arranged in said first end and/or in said second end of said handle.