H. P. Lovecraft is best known and most studied because of his Cosmic Horror, and his developments on the element of horror in Weird Literature; in fact, his pioneering Supernatural Horror in Literature, published in 1927, is commonly accepted as one of the most important essays at this respect. However, one of the priorities of this article is to prove that some of his works belong to the genre named Science Fiction. His approach to sciences since his early childhood, and his "repulsion" against to any nonscientific materialistic beliefs drove him to create Science Fiction, not exclusively horror stories. In fact, H. P. Lovecraft is the most important, and one of the earliest introducers of horror into Science Fiction. For that reason, and after an introduction stating how his aesthetic and rhetoric views, as well as several issues concerning his political and social ideas of the America where he was born, interfered with his popularity and the recognition of his work until almost the last quarter of the 20th century, this study shall show relevant poetic elements, as well as themes and motifs -extracted from Science Fiction. History. Science, Vision, written by Scholes, R., & Rabkin, E. S. (1977), one of the pillars in Science Fiction studies-, scientific theories and hypothesis employed, and facts found in Lovecraft's work. The analysis will focus on the manner in which such components can be observed specifically in seventeen of his writings, although they also arise in some other works that belong to Weird literature and Space Opera. This analysis also compels to understand part of his stories as the beginning of a new sub-genre that could be named Science Fiction-Horror, of which he may surely be the initiator and one of the modern masters, next to Richard Matheson and Stephen Edwin King. Finally, the end of the present article will add some more details in order to corroborate this thesis with reference to these seventeen texts.