It means "son of Tiede" or "son of Tiedje".
Tiede or Tiedje is a diminutive form of the German given name Theodor, which means "gift of God".
Therefore, the surname Tietjen signifies someone who is descended from a man named Tiede or Tiedje (a diminutive of Theodor).
It is important to note that:
* The surname Tietjen is primarily found in northern Germany, particularly in Lower Saxony and Schleswig-Holstein.
* It is also found in other countries with German ancestry, such as the United States and Canada.
While the exact meaning of the name is clear, the specific origin of the individual Tiede or Tiedje who the surname is descended from is usually lost to time.