A monitoring device for tyres for vehicle wheels, comprising: - an electronic unit; - a connecting member configured for constraining said electronic unit to a tyre, said connecting member comprising: • a first and a second base portion (31, 32), mutually separated by a separation region, each of said base portions (31, 32) having a respective base surface (31 a, 32a) associable with an inner surface of a tyre; • a housing portion associated with said base portions (31, 32) defining, in cooperation with said base portions (31, 32), a cavity for housing said electronic unit, wherein said housing portion comprises at least one side structure, configured for exerting a containing action with respect to movements of said electronic unit carried out in a direction orthogonal to a major extension direction of said separation region, wherein said cavity is confined at least by a side surface defined by an inner surface of said side structure, wherein said electronic unit comprises at least one sensor, at least one antenna and a holding body for housing at least said sensor and antenna, wherein said holding body has at least one side surface, wherein, when said electronic unit is inserted in said cavity: • the side surface of the holding body faces the side surface of the cavity, • the sizes of said cavity and of said holding body are such as to form at least one first gap at least partly separating the side surface of the holding body and the side surface of the cavity, characterized in that said side structure is configured for exerting a containing action at least relative to movements of said electronic unit carried out in the major extension direction of said separation region. - an electronic unit; - a connecting member configured for constraining said electronic unit to a tyre, said connecting member comprising: • a first and a second base portion (31, 32), mutually separated by a separation region, each of said base portions (31, 32) having a respective base surface (31 a, 32a) associable with an inner surface of a tyre; • a housing portion associated with said base portions (31, 32) defining, in cooperation with said base portions (31, 32), a cavity for housing said electronic unit, wherein said housing portion comprises at least one side structure, configured for exerting a containing action with respect to movements of said electronic unit carried out in a direction orthogonal to a major extension direction of said separation region, wherein said cavity is confined at least by a side surface defined by an inner surface of said side structure, wherein said electronic unit comprises at least one sensor, at least one antenna and a holding body for housing at least said sensor and antenna, wherein said holding body has at least one side surface, wherein, when said electronic unit is inserted in said cavity: • the side surface of the holding body faces the side surface of the cavity, • the sizes of said cavity and of said holding body are such as to form at least one first gap at least partly separating the side surface of the holding body and the side surface of the cavity, characterized in that said side structure is configured for exerting a containing action at least relative to movements of said electronic unit carried out in the major extension direction of said separation region. • a first and a second base portion (31, 32), mutually separated by a separation region, each of said base portions (31, 32) having a respective base surface (31 a, 32a) associable with an inner surface of a tyre; • a housing portion associated with said base portions (31, 32) defining, in cooperation with said base portions (31, 32), a cavity for housing said electronic unit, wherein said housing portion comprises at least one side structure, configured for exerting a containing action with respect to movements of said electronic unit carried out in a direction orthogonal to a major extension direction of said separation region, wherein said cavity is confined at least by a side surface defined by an inner surface of said side structure, wherein said electronic unit comprises at least one sensor, at least one antenna and a holding body for housing at least said sensor and antenna, wherein said holding body has at least one side surface, wherein, when said electronic unit is inserted in said cavity: • the side surface of the holding body faces the side surface of the cavity, • the sizes of said cavity and of said holding body are such as to form at least one first gap at least partly separating the side surface of the holding body and the side surface of the cavity, characterized in that said side structure is configured for exerting a containing action at least relative to movements of said electronic unit carried out in the major extension direction of said separation region.