Lothrop Withington (January 31, 1856 – May 7, 1915) was a well-known American genealogist, historian, and book editor who was killed in the sinking of the RMS Lusitania.

Born in Newburyport, Massachusetts to Nathan Noyes Withington (1828–1914) and Elizabeth (Little) Withington (1828–1912), he was involved with research and editing of publications on certain aspects of the American Revolutionary War but best known was his extensive genealogical research that included the publication of immigrant ships' passenger lists and the like.

On October 14, 1892, in London, he married Caroline Augusta Lloyd, a sister of Henry Demarest Lloyd. They had no children.

In May 1915, he was returning to his work in the United Kingdom but lost his life on board the RMS Lusitania when it was attacked and sunk by German U-boat U-20.

He had no children at the time of his death. His grandnephew (a brother's grandson) was Lothrop Withington Jr., the progenitor of goldfish swallowing.