An adjustment device for a cable management arm includes a first support member, a second support member, and a positioning member. The first support member has a plurality of holes. The second support member is movably connected to the first support member and has a through hole. The positioning member is fixed to the second support member and has a positioning portion and a resilient section connected to the positioning portion. When in use, the positioning portion partially extends through the through hole, selectively extends through one of the holes, and then is positioned by a force of the resilient section, such that the extension-length of the second support member relative to the first support member is adjusted.