The Joestar Birthmark is a clearly, five-pointed, star-shaped mark appearing in the region of the back of the left shoulder on all JoJos or individuals of or in certain ways connected to the Joestar bloodline.
In Stone Ocean, after inheriting Made in Heaven, Enrico Pucci and Weather Report also receive the mark, as the Stand partially originated with DIO's Bone; and as brothers, they connect and affect one another in the same way DIO does Joseph, Holy and Jotaro in Stardust Crusaders. Once Emporio Alniño enters the new universe, he discovers that Irene possesses the mark, helping him identify Jolyne as surviving in a new form.
As JoJos, Joestars, or members of DIO's progeny, it may be assumed that George Joestar I, George Joestar II, Josuke Higashikata, Donatello Versus, Nicholas Joestar, and Johnny Joestar all have the birthmark as well.
In the new universe, more specifically JoJolion, the protagonist's birthmark factors in his Stand ability, and it is eventually revealed that Holy Joestar-Kira, Yoshikage Kira, Kyo Nijimura, and Josefumi Kujo also possess it.