A magnetic tape cartridge includes a magnetic tape wound around a reel, in which the magnetic tape has a first region, which is a predetermined region influenced by a width change of the magnetic tape accompanied by deformation of the reel due to winding of the magnetic tape, and a second region, which is a region on an outer peripheral side of the reel with respect to the first region, and regarding a degree of elongation in a longitudinal direction of the magnetic tape, the degree of the first region is equal to or larger than the degree of the second region.