Thaddeus Mason Harris (July 7, 1768 – April 3, 1842) was a Harvard librarian, Unitarian minister and author in the early 19th Century.

[1] Harris was named after his maternal grandfather Thaddeus Mason, Harvard University class of 1728 and secretary to Jonathan Belcher.

At the end of his teaching stint in Worcester, Harris was offered an appointment as secretary to George Washington, but contracted smallpox, and his recovery time prevented him from taking the post.

[5] The Antiquarian Society holds original copies of a significant number of Harris's published works, as well as some manuscript items from his personal papers.

Thaddeus Harris was a long-standing member and frequent visitor to the Boston Athenæum private library and Nathaniel Hawthorne once claimed to have encountered his ghost there.