The Lancaster County Jail is a public building in Lancaster, South Carolina. It was built in 1823 and is best known for having notable innovations in jail design.

Constructed after the design of Robert Mills, a well known expert in penal institutions architecture, he Lancaster County Jail is said to be one of the most significant works of the man. Unlike other structures of the sort at the time, Mills traded dungeons for cells, so air circulation could be better and more efficiently achieved. It also practiced a method that separated debtors from individuals with heavier crimes.

The Lancaster County Jail was declared as a National Historic Landmark in November 1973, mostly for its unique and original architecture. It is also a contributing property to the Lancaster Downtown Historic District.