| K. | |
| Author: | Mary Roberts Rinehart |
| Country: | United States |
| Language: | English |
| Genre: | Novel |
| Publisher: | Houghton Mifflin |
| Release Date: | 1915 |
| Media Type: | Print (Hardback) |
| Pages: | 410 |
| Oclc: | 39111410 |
K. is a 1915 crime novel by the American writer Mary Roberts Rinehart (1876–1958) set in post-Victorian era Allegheny, Pennsylvania, which has been a part of the city of Pittsburgh since 1907.
K. is a romance set in the industrial Victorian era. The novel tells the story of Sidney; when Sidney takes in a boarder with the initial "K", her life becomes entwined with the mystery surrounding "K". Lies and intrigue surround Sidney.
A film version of the novel K – The Unknown starring Virginia Valli and Percy Marmont, was released on 17 November 1924 by Universal Pictures as a "Universal Jewel" release. Another film version was released under the title The Doctor and the Woman in 1918.