Doctor Syn is a 1915 novel by Russell Thorndike, the first in a series about Christopher Syn, an 18th-century clergyman who secretly leads a double life as the fearsome smuggler Captain Clegg. Set in the Romney Marsh area of Kent, England, the story blends historical adventure, piracy, smuggling, and Gothic mystery.

Dr. Christopher Syn, a seemingly respectable vicar of Dymchurch, hides a dark past. Once a feared pirate and smuggler, he now leads a secret band of smugglers under the disguise of "The Scarecrow", a masked figure who defies the King's men and helps the local people evade heavy taxes. The novel follows his battles with government excise men, rival outlaws, and those who might expose his identity.