Tamsin is a novel by Peter S. Beagle.

Tamsin is also a feminine version of the name Thomas. See Thomasina for more information.

The novel tells of a teenage girl named Jenny Gluckstein, a social misfit uprooted from her life in New York City when her divorced mother marries an Englishman. At first Jenny resents her stepfather, his two sons, and the family's new home in an isolated manor house in Dorset. Soon, though, Jenny discovers that the house is haunted by the spirit of Tamsin Willoughby, a young woman who died in the house three hundred years before. Tamsin introduces her to the night world of boggarts, the Billy Blind, and the Pooka, before a three-hundred-year-old ghost, Judge Jeffreys, is finally awakened and begins to pursue her. The story ends with Judge Jeffreys' three-hundred-year-old magic being defeated by an older magic, that of the old lady of the elder tree. This ending is similar to that at the end of Beagle's The Folk of the Air.