An assembly/joining device (10) for parts or components of modular furniture and furnishing accessories of the shelf units, bookcases, cupboards, bed frames type and the like, including a pin (12) and a bushing (14) inserted in dedicated seats made in the components of the furniture to be joined and attached rigidly one with respect to the other, the pin (12) which includes a ferrule (24) elastically retractable internally to the pin apt to couple with the bushing (14) and co-operating with guide elements of the bushing (14) for a coupling/uncoupling between the pin (12) and bushing (14) during assembly/disassembly of the furniture parts.