The Thin Blue Line is a 1988 documentary film concerning the murder of a Texas police officer who had stopped a car for a routine traffic citation. The police are presented with two suspects, one a local underaged boy with a criminal record (David Ray Harris , a boy who returned to his hometown boasting that he had murdered a policeman) and the other a 38-year-old taciturn drifter with no criminal record whatsoever (Randall Dale Adams ). The documentary presents testimony suggesting that the police altered, fabricated, and suppressed evidence to convict the man they wanted to be guilty, in spite of evidence to the contrary.

The film was directed by Errol Morris, (who had, incidentally, spent some years before the filming as a private investigator), and cost over $1 million U.S. to make. It was entered into evidence in the federal appeal but, since it was marketed as a "nonfiction" film rather than a documentary, it was not entered into evidence in the case itself. For the same reason, the film was disqualified from the Academy Award for Best Documentary. Nonetheless, Adams was finally granted a retrial and released after eleven years in prison. (Shortly after his release, he sued Morris, claiming that Morris had gotten rich off the film--which proved to be untrue).

Harris had testitfied that he and Adams were together in a car and that Adams committed the murder. He later recanted this testimony; however, he never admitted guilt in a judicial setting and was never charged in the case. Harris was executed in 2004 for another murder which occurred during an attempted abduction in 1985.

The film's title comes from the prosecutor's comment to the trial judge that the police are the "thin blue line" separating society from anarchy. (The thin blue line is a common symbol to law enforcement: a blue line on a black background, with the black background standing for those who gave their lives in the line of duty protecting others and the blue line representing the brave who still serve the community.)

In December 2001, the United States' National Film Preservation Foundation declared the film "culturally significant" and announced that it would be one of the 25 films selected that year for preservation in the National Film Registry at the Library of Congress, bringing the total at the time to 325.