The paper is an examination and critique of the philosophy of science fiction horror of seminal American horror, science fiction and fantasy writer H.P. Lovecraft (1890-1937). Lovecraft never directly offers a philosophy of science fiction horror. However, at different points in his essays and letters, he addresses genres he labels “interplanetary fiction”, “horror”, “supernatural horror”, and “weird fiction”, the last being a broad heading covering both supernatural fiction and science fiction. Taken together, a philosophy of science fiction horror emerges. Central to this philosophy is the juxtaposition of the mysterious, unnatural and alien against a realistic background, in order to produce the emotion that Lovecraft calls “cosmic fear”. This background must not only be scientifically accurate, but must accurately portray human psychology, particularly when humans are faced with the weird and alien. It will be argued that Lovecraft’s prescriptions are overly restrictive and would rule out many legitimate works of science fiction horror art. However, he provides useful insights into the genre.