A fruit-on-tree sizing device, characterised in that it comprises: - a lower annular disc that has an inner compartment and a guide around its perimeter thereof, intended for rotating in one direction, - first flexible arms with curved configuration, wherein each first flexible arm starts internally from the lower annular disc and ends at an inner end, which together with the inner ends of the other first flexible arms define a lower opening intended for housing a fruit, wherein said inner end has a first hole, - an upper annular disc, parallel to the lower annular disc and intended for rotating in the opposite direction to the lower annular disc, and which comprises a first attachment element on the face facing the lower annular disc, - second flexible arms with curved configuration, wherein each second flexible arm starts internally from the upper annular disc and ends at an inner end, which together with the inner ends of the other second flexible arms define an upper opening intended for housing the fruit, wherein said inner end has a second hole - at least one bolt, connected to the upper annular disc and intended for moving along the guide when the upper annular disc rotates with respect to the lower annular disc, holding the lower annular disc and the upper annular disc together and enabling the movement of one with respect to the other, - means for connecting the arms each one connected to the first hole of a first flexible arm and to the second hole of a corresponding second flexible arm, - a second attachment element associated with the first attachment element, - a reading module, which has a first reading element associated with the second attachment element, and a second reading element located in the inner compartment of the lower annular disc, the first reading element being located in correspondence with the second reading element, the reading module being capable of measuring the movement of the upper annular disc with respect to the lower annular disc, which is representative of the growth of the fruit and therefore of the size thereof, - communication means, located inside the inner compartment, intended for sending the measurement obtained by the reading module to an external device, - at least one battery for powering the device, positioned inside the inner compartment. - a lower annular disc that has an inner compartment and a guide around its perimeter thereof, intended for rotating in one direction, - first flexible arms with curved configuration, wherein each first flexible arm starts internally from the lower annular disc and ends at an inner end, which together with the inner ends of the other first flexible arms define a lower opening intended for housing a fruit, wherein said inner end has a first hole, - an upper annular disc, parallel to the lower annular disc and intended for rotating in the opposite direction to the lower annular disc, and which comprises a first attachment element on the face facing the lower annular disc, - second flexible arms with curved configuration, wherein each second flexible arm starts internally from the upper annular disc and ends at an inner end, which together with the inner ends of the other second flexible arms define an upper opening intended for housing the fruit, wherein said inner end has a second hole - at least one bolt, connected to the upper annular disc and intended for moving along the guide when the upper annular disc rotates with respect to the lower annular disc, holding the lower annular disc and the upper annular disc together and enabling the movement of one with respect to the other, - means for connecting the arms each one connected to the first hole of a first flexible arm and to the second hole of a corresponding second flexible arm, - a second attachment element associated with the first attachment element, - a reading module, which has a first reading element associated with the second attachment element, and a second reading element located in the inner compartment of the lower annular disc, the first reading element being located in correspondence with the second reading element, the reading module being capable of measuring the movement of the upper annular disc with respect to the lower annular disc, which is representative of the growth of the fruit and therefore of the size thereof, - communication means, located inside the inner compartment, intended for sending the measurement obtained by the reading module to an external device, - at least one battery for powering the device, positioned inside the inner compartment.