Grant Thiessen (born in 1947 in Winnipeg, Manitoba) is a Canadian bibliographer and bookseller working primarily in the area of science fiction.
Thiessen's bibliographic fanzine, The Science Fiction Collector, has been collected into three hardbound volumes by Pandora's Books.
The volumes also contain biographies of individiual authors including James H. Schmitz, Jack Williamson, E. C. Tubb, A. E. van Vogt, and Charles L. Harness.
Thiessen has also published "The Tanelorn Archives", a bibliography of Michael Moorcock, and has made many contributions to bibliographic reference works, magazines, and price guides, as well as producing several hundred book catalogs.
Grant continued to sell books and write computer software, as BookIT Enterprises Inc, until his retirement in early 2022.