The invention relates to a quick-assembly breech plug for muzzleloader firearms, allowing a quick assembly and disassembly and being extremely practical, simple to manufacture and having a low cost, wherein said breech plug comprises a cylindrical body (1), configured to be internally coupled in a rear end (10') of a barrel (10), and a head (6) configured to define a limit coupling position of the breech plug inside the barrel (10), wherein the cylindrical body (1) externally comprises a male thread (2) having at least two starts located close to a front end (3) of the breech plug, opposite the head (6), according to a plane perpendicular to an axis of revolution of the cylindrical body (1), said male thread (2) being configured to screw into a female thread (10'') located internally in the rear end (10') of the barrel (10), preferably 4 screw ridges.