Hazrat Mahdi (as) will have a largish, dark brown, leaf-shaped mark between his shoulders (on his right chest)

… He (Hazrat Mahdi [as]) HAS A MARK RESEMBLING A LEAF BENEATH HIS SHOULDERS [ON HIS RIGHT CHEST].

(Mu’jam Ahadith al-Imam al-Mahdi, Volume 3, p. 283, hadith 766)

...Imam Muhammad Bakr (ra) or Imam Ja’far Sadiq (ra) said, “The blessed one [Hazrat Mahdi (as)] has two signs, the Qaim (as) will have a black mark on his head [a mole on his cheek] and a black mark resembling a basil leaf between his shoulders [on his right chest]...”

(al-Ghaybah al-Numani)