The Hideous Sun Demon (sometimes billed as The Sun Demon, or in the UK as Blood on His Lips) is a 1958 American science fiction horror film produced, directed, and cowritten by Robert Clarke, who also starred in the title role. It also stars Patricia Manning, Nan Peterson, Patrick Whyte, and Fred La Porta. The film focuses on a scientist (portrayed by Clarke) who is exposed to a radioactive isotope and soon finds out that it comes with horrifying consequences.

The film was inspired by the financial success of The Astounding She-Monster, in which Clarke had starred earlier that year. The crew was made up of University of Southern California film students, while the cast consisted of unknowns in addition to Clarke's family and friends. Shooting took place under three different cinematographers over 12 consecutive weekends. Originally budgeted at $10,000, the film ended up costing $50,000. Distributed by Clarke's own Pacific International Pictures, The Hideous Sun Demon premiered on August 29, 1958 as part of a double bill with Roger Corman's Attack of the Crab Monsters. The film received mostly negative reviews upon its release, but has since become a cult film and has been referenced and parodied many times. An unauthorized sequel, the 1965 short film Wrath of the Sun Demon, was produced by Donald F. Glut. Two redubbed versions of the original film have been released: the comedic Hideous Sun Demon: Special Edition and What's Up, Hideous Sun Demon (also known as Revenge of the Sun Demon), the latter of which was produced with Clarke's permission.