Actuator device for a bicycle gearshift, comprising: - an actuation kinematic mechanism configured to deform to move a derailleur of the gearshift, the actuation kinematic mechanism comprising a first body configured to be fixed to a bicycle frame, a second body configured to support the derailleur of the gearshift and a pair of articulating connecting rods (102, 104) that connect the first body and the second body; - a drive member configured to control the deformation of the actuation kinematic mechanism, the drive member comprising a motor fixedly connected to one of the first body and the second body, and a motion transmission element having a predetermined rotation axis and set in rotation by the motor; - a snap mechanism arranged between said motion transmission element and a connecting rod of said pair of connecting rods (102, 104) and comprising a first member rotating integrally with said motion transmission element and provided with a coupling seat and a second member fixedly connected to said connecting rod and comprising a coupling element that, when the actuator device is in a first operative engagement configuration, is housed at least partially in said coupling seat and allows the transfer of the rotation from said first member to said second member and that, when the actuator device is in a second operative release configuration, is not housed in said coupling seat and allows the free rotation of said first member with respect to said second member, said first operative configuration being defined when the actuator device is subjected to a force lower than a predetermined threshold force and said second operative configuration being defined when the actuator device is subjected to a force greater than said predetermined threshold force; wherein said second member comprises: - a first cavity extending along a first direction coinciding with said predetermined rotation axis and a second cavity extending along a second direction substantially perpendicular to the first direction; wherein said first cavity houses at least partially said first member and said second cavity houses a pre-loaded elastic element pushing against said coupling element along said second direction towards said first member so that, when the actuator device is in said first operative configuration, said coupling element is pushed into said coupling seat; wherein the motion transmission element: i) is defined by a motion transmission shaft of the motor, the first member being coaxially fixed to a free end portion of said motion transmission shaft; or ii) comprises a grooved pin housed in a grooved seat defined in said motor coaxially to said predetermined rotation axis, the first member being coaxially fixed to a free end portion of said grooved pin. - an actuation kinematic mechanism configured to deform to move a derailleur of the gearshift, the actuation kinematic mechanism comprising a first body configured to be fixed to a bicycle frame, a second body configured to support the derailleur of the gearshift and a pair of articulating connecting rods (102, 104) that connect the first body and the second body; - a drive member configured to control the deformation of the actuation kinematic mechanism, the drive member comprising a motor fixedly connected to one of the first body and the second body, and a motion transmission element having a predetermined rotation axis and set in rotation by the motor; - a snap mechanism arranged between said motion transmission element and a connecting rod of said pair of connecting rods (102, 104) and comprising a first member rotating integrally with said motion transmission element and provided with a coupling seat and a second member fixedly connected to said connecting rod and comprising a coupling element that, when the actuator device is in a first operative engagement configuration, is housed at least partially in said coupling seat and allows the transfer of the rotation from said first member to said second member and that, when the actuator device is in a second operative release configuration, is not housed in said coupling seat and allows the free rotation of said first member with respect to said second member, said first operative configuration being defined when the actuator device is subjected to a force lower than a predetermined threshold force and said second operative configuration being defined when the actuator device is subjected to a force greater than said predetermined threshold force; wherein said second member comprises: - a first cavity extending along a first direction coinciding with said predetermined rotation axis and a second cavity extending along a second direction substantially perpendicular to the first direction; i) is defined by a motion transmission shaft of the motor, the first member being coaxially fixed to a free end portion of said motion transmission shaft; or ii) comprises a grooved pin housed in a grooved seat defined in said motor coaxially to said predetermined rotation axis, the first member being coaxially fixed to a free end portion of said grooved pin.