Actuator device for a bicycle gearshift, comprising: - an actuation linkage including a first body configured to be associated with a bicycle frame, a second body configured to be associated with a derailleur of the bicycle gearshift and a pair of articulating connection rods (102, 104) that connect the first body and the second body; - a drive member configured to control the deformation of the actuation linkage and comprising a motor fixedly connected to one of the first body and second body and a motion transmission element configured to be set in rotation by the motor about a rotation axis; - a fastening/release mechanism operatively arranged between said motion transmission element and a connection rod of said pair of connection rods (102, 104) and comprising: - a first member rotatable about the rotation axis as a unit with said motion transmission element; - a second member rotatable about the rotation axis together with said connection rod; - an elastic element wherein the fastening/release mechanism is selectively configurable in a fastening operative condition in which the first member is rotatable about the rotation axis as a unit with the second member, and in a release operative condition in which the first member and the second member can rotate with respect to one another about the rotation axis, said fastening operative condition being defined when the fastening/release mechanism is subjected to a stress lower than said predetermined pre-load stress and said release operative condition being defined when the fastening/release mechanism is subjected to a stress greater than said predetermined pre-load stress, characterized in that the elastic element is operatively arranged between the first member and the second member and the elastic element exerts a predetermined pre-load stress on the first member and on the second member. - an actuation linkage including a first body configured to be associated with a bicycle frame, a second body configured to be associated with a derailleur of the bicycle gearshift and a pair of articulating connection rods (102, 104) that connect the first body and the second body; - a drive member configured to control the deformation of the actuation linkage and comprising a motor fixedly connected to one of the first body and second body and a motion transmission element configured to be set in rotation by the motor about a rotation axis; - a fastening/release mechanism operatively arranged between said motion transmission element and a connection rod of said pair of connection rods (102, 104) and comprising: - a first member rotatable about the rotation axis as a unit with said motion transmission element; - a second member rotatable about the rotation axis together with said connection rod; - an elastic element wherein the fastening/release mechanism is selectively configurable in a fastening operative condition in which the first member is rotatable about the rotation axis as a unit with the second member, and in a release operative condition in which the first member and the second member can rotate with respect to one another about the rotation axis, said fastening operative condition being defined when the fastening/release mechanism is subjected to a stress lower than said predetermined pre-load stress and said release operative condition being defined when the fastening/release mechanism is subjected to a stress greater than said predetermined pre-load stress, characterized in that the elastic element is operatively arranged between the first member and the second member and the elastic element exerts a predetermined pre-load stress on the first member and on the second member. - a first member rotatable about the rotation axis as a unit with said motion transmission element; - a second member rotatable about the rotation axis together with said connection rod; - an elastic element