A tattoo is a physical mark on a person that is applied through various means, and is meant to be a permanent mark. The Bible forbids God's people from putting such marks on their own skin, as mentioned in the book of Leviticus.

In the Left Behind series, the "mark of loyalty" that was enforced by Nicolae Carpathia included the tattoo mark of the Potentate's name or initials or the region code of that person residing within the Global Community that is applied on the person's right hand or forehead along with the biochip. Orcus during his brief visit outside of Petra offered the "mark of loyalty" through supernatural means to the unsaved who ventured out of Petra to see him. After Chang Wong was rescued from New Babylon, Tsion Ben-Judah along with Chaim Rosenzweig and a few of the Petra elders prayed to God to have the "mark of loyalty" removed from Chang, and thus it was granted.

Mainyu Mazda, a Middle Eastern black market trader, is a man who decorated himself with tattoos, an M for each murder he committed. He also claimed that his "mark of loyalty", which wasn't his actual region code, was one he had applied by someone other than a Global Community loyalist.