Device for applying a closure on a mouth of a container, comprising: a capping or sealing head bearing said closure; a plate-shaped body elastically coupled to said capping or sealing head; at least a centring plug integrally constrained to said capping or sealing head and slidably inserted in a slot obtained in said plate-shaped body, said centring plug having a conical end and said slot being counter-shaped so as to house said conical end; activating means operatively active on said plate-shaped body for shifting it in such a way that the device passes from a first configuration in which the capping or sealing head is spaced apart with respect to said mouth and the conical end engages said slot, to a second configuration in which the capping or sealing head is brought close to said mouth and the conical end at least partly disengages said slot; a blocking system for blocking the reciprocal sliding of the centring plug with respect to said plate-shaped body, wherein said blocking system comprises: a first cylindrical element having an end which is constrained to a fixed portion of the device and the other end which is free, said first cylindrical element being slidably inserted in a further slot obtained in said plate-shaped body; a second cylindrical element having an end which is constrained to said capping or sealing head and the other end which is free, the free ends (11b, 12b) of said cylindrical elements (11, 12) being shaped so that in a third configuration of the device they are brought close to one another and reciprocally abut in such a way as to define a joint. a capping or sealing head bearing said closure; a plate-shaped body elastically coupled to said capping or sealing head; at least a centring plug integrally constrained to said capping or sealing head and slidably inserted in a slot obtained in said plate-shaped body, said centring plug having a conical end and said slot being counter-shaped so as to house said conical end; activating means operatively active on said plate-shaped body for shifting it in such a way that the device passes from a first configuration in which the capping or sealing head is spaced apart with respect to said mouth and the conical end engages said slot, to a second configuration in which the capping or sealing head is brought close to said mouth and the conical end at least partly disengages said slot; a blocking system for blocking the reciprocal sliding of the centring plug with respect to said plate-shaped body, wherein said blocking system comprises: a first cylindrical element having an end which is constrained to a fixed portion of the device and the other end which is free, said first cylindrical element being slidably inserted in a further slot obtained in said plate-shaped body; a second cylindrical element having an end which is constrained to said capping or sealing head and the other end which is free, the free ends (11b, 12b) of said cylindrical elements (11, 12) being shaped so that in a third configuration of the device they are brought close to one another and reciprocally abut in such a way as to define a joint. a first cylindrical element having an end which is constrained to a fixed portion of the device and the other end which is free, said first cylindrical element being slidably inserted in a further slot obtained in said plate-shaped body; a second cylindrical element having an end which is constrained to said capping or sealing head and the other end which is free, the free ends (11b, 12b) of said cylindrical elements (11, 12) being shaped so that in a third configuration of the device they are brought close to one another and reciprocally abut in such a way as to define a joint.