William Miller (1782 - 1849) was an American baptist preacher, hence: millerists , millerites. His preaching spurred the founders of Seventh-day Adventism to look closer into prophecy and other doctrines and organize that church.

Miller began to preach in 1831. Based on Daniel 8-9, Miller believed that 2300 years would pass from the time of the commandment to restore and rebuild Jerusalem to the Second Coming of Jesus. The commandment was given to the scribe Ezra by Artaxerxes I of Persia in the seventh year of his reign (i.e. 457 B.C.), and is detailed in Ezra 7:12-26. Thus, 1843 became the year of Christ's return. As the appointed year grew closer, Miller first specified 21 March 1843 as the date. After that date passed, further study revealed that there was no year zero in the Gregorian calendar, and October 22, 1844 was the actual date.

Christ's failure to appear in 1844 has come to be know as the Great Disappointment. It is estimated that the Millerites, as they came to be known, numbered nearly 50,000. Miller recorded his personal disappointment in his memoirs: "Were I to live my life over again, with the same evidence that I then had, to be honest with God and man, I should have to do as I have done I confess my error, and acknowledge my disappointment." (Memoirs of William Miller, Sylvester Bliss, p. 256).

Seventh-day Adventists maintain that Christ went into the second apartment of the heavenly sanctuary on October 22, 1844 to begin the investigative judgment of both righteous and wicked to see who is actually ready to go to heaven. This investigative judgment takes place prior to his actual return to earth, which is they believe to be very soon.

Members of the Bahá'í Faith however, believe that the prophecy was fulfilled by the coming of their prophet the Báb who heralded the coming of the return of Christ, Bahá'u'lláh. October 1844 is when the Báb started to make his public claim and began the Bábí religion.

Another William Miller was Governor of North Carolina, 1814-1817. See William Miller (politician)