Britain's most prominent atheist attacked the core tenets of Mr Romney's religion, saying that the Church of Latter Day Saints' founding prophet was "a fraud" and that the presidential contender was "too stupid to see it".
"No matter how much you agree with Romney's economic policy, can you really vote for such a massively gullible fool?" asked Prof Dawkins during an outburst on Twitter that lasted several hours.
The Oxford academic focused his criticism on the Church's belief that its founder, Joseph Smith, was visited by an angel in 1820s New York, who guided him to a set of golden plates buried in a hill.
Smith claimed to have translated runes engraved on the plates, and compiled them into the Book of Mormon. The text describes how Jesus Christ appeared in the United States after the Crucifixion and how Adam and Eve went to the site of present-day Missouri after being expelled from the Garden of Eden.
Prof Dawkins said Smith was a "19th century charlatan" and that the Book's text appeared to be written in a falsified ancient tone, unlike the Bible and Koran which were "written in the language of their time".