Tom Cruise: An Unauthorized Biography is an unauthorized biography of actor Tom Cruise, written by Andrew Morton. The book was published in the United States in hardcover format on January 15, 2008, by St. Martin's Press, with a first printing of 400,000 copies, and an audio format on five CDs by Macmillan Audio.

Cruise's lawyers and the Church of Scientology have released several statements which question the truthfulness of assertions made by Morton in the book. In an official 15-page statement released to the press, the Church called the book "a bigoted, defamatory assault replete with lies". The book was not published in the United Kingdom or New Zealand due to strict libel laws in those countries. Although initially not published in Australia, it was later published there and became popular.

The book hit number one on Amazon.com's list of top sellers three days after it was published, and was number one on The New York Times Best Seller list one week after publication. It was the number one bestseller in Australia for publisher AbeBooks in 2008. The book received mixed and critical reviews in The New York Times. The San Jose Mercury opined that it should be taken "with the proper grain of salt".

Morton's book describes Cruise's relationship with Katie Holmes, his sexuality, and Cruise and Holmes' beliefs. Morton also asserts in the book that Holmes had to "audition" for the status of Cruise's girlfriend, and won the part over other actresses. The book also discusses details about Cruise's marriages to Mimi Rogers and Nicole Kidman, his relationship with Penélope Cruz, his behavior on The Oprah Winfrey Show, and his denunciation of Brooke Shields for using anti-depressants to help her cope with postpartum depression. Morton consulted a former senior Scientologist, who asserted that Kidman's lawyer had advised her not to publicly discuss Scientology or to speak out against it, even though she "hated" Scientology when Cruise left her in 2000. Morton's source stated "I told the lawyer if she wants to stay with the children she will have to be quiet and not speak out about Scientology." Morton writes that Holmes joined Scientology in June 2006, and agreed that "if she or any of her children were ever to suffer mental or terminal illness, they must turn only to Scientology's treatments". Morton asserts that model Sofia Vergara stopped seeing Cruise in 2005 weeks before he met Holmes, and Vergara felt "she had been deliberately targeted not only as a possible bride for Tom, but as a high-profile Scientology recruit who would be an alluring figurehead for a future recruitment drive in Latin America".

Morton asserts in the book that Cruise is the "de facto second-in-charge" in the Church of Scientology. When asked by the Associated Press what evidence he had about this, Morton stated "Scientology would be a shadow of what it is today if it had not been for the involvement of Tom Cruise. He has been the poster boy. More than that, he has been recruiting fellow celebrities - people like Will Smith, Jada Pinkett Smith. ... More than that, he's been the front man for the organization."

Morton writes that Church of Scientology leader David Miscavige consults Cruise on "every aspect of planning and policy," that Miscavige joined Cruise and Holmes on their 2006 honeymoon in the Maldives, and that Church of Scientology officials select many of the employees that staff Cruise's mansion. According to Morton, Miscavige invited Cruise to the Church of Scientology's Gold Base in Hemet, California in 1989.

Kidman and Cruise were invited to Gold Base in 1990 after spending time together on the set of Days of Thunder. Morton writes "When Tom confided to the Scientology leader about the couple's fantasy of running through a meadow of wild flowers together, his friend apparently decided to make his dream come true." Morton writes that around the same time Cruise was beginning his relationship with Rogers, Miscavige made an announcement at a Church of Scientology rally, "The most important recruit ever is in the process of being secured. His arrival will change the face of Scientology forever."

According to Morton, he began researching Tom Cruise after publishing his best-selling book on Princess Diana, Diana: Her True Story. Morton stated that he became interested in writing about the actor after watching Cruise jump on Oprah's couch during a May 23, 2005 appearance on The Oprah Winfrey Show, and lecture Matt Lauer on his beliefs regarding psychiatry during a June 24, 2005 appearance on The Today Show "I wondered: what is a 43-year-old man, who has been married twice before to women he has said were the loves of his life, with his son and daughter watching, doing behaving in this fashion — all because of Katie Holmes, a woman he has known for a matter of days? ... What was going on? I wanted to know more."