The flag of the University displays the school's shield and motto. The open book is inscribed with "Vet. Nov. Testamentum" (Old and New Testament). The motto, "Dei Sub Numine Viget," translates as "Under the Protection of God She Flourishes."

The orange of the flag stands for William III of Orange, Prince of Nassau, and King of England. Black was first used as Princeton's other color in 1867, when the sophomore baseball team used black ink to write class numerals on their orange ribbons.