System (S) 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 (R), able to rotate around said axis (R), clockwise and counterclockwise, and provided with a first end and a second end , a first plurality of weights (P1), 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 (P2), 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 (S) 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 (U) able to receive said data sent by said at least one detection device and to associate to said data a weight of said first (P1) and second (P2) 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 (R), able to rotate around said axis (R), clockwise and counterclockwise, and provided with a first end and a second end , a first plurality of weights (P1), 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 (P2), 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 (R), able to rotate around said axis (R), clockwise and counterclockwise, and provided with a first end and a second end , a first plurality of weights (P1), 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 (P2), 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 (S) 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 (U) able to receive said data sent by said at least one detection device and to associate to said data a weight of said first (P1) and second (P2) 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 . <EOS>