Marian Longstreth Thayer (November 9, 1872 – April 14, 1944) was an American socialite and survivor of the sinking of the RMS Titanic. She was the wife of John Borland Thayer II, a Director and Second Vice President of the Pennsylvania Railroad Company, and the mother of John Borland "Jack" Thayer III. In 1912, all three of them, along with their maid Margaret Fleming, were passengers on RMS Titanic maiden voyage when it struck an iceberg and sank. Marian, Jack III, and Fleming all survived, but John II died.