The Thin Blue Line is a 1988 American documentary film by Errol Morris, about the trial and conviction of Randall Dale Adams for the 1976 shooting of Dallas police officer Robert W. Wood. Morris became interested in the case while doing research for a film about Dr. James Grigson, a psychiatrist known in Texas as "Dr. Death" for testifying with "100 percent certainty" of a defendant's recidivism in many trials, including that of Randall Adams. The film centers around the "inconsistencies, incongruities and loose ends" of the case, and Morris, through his investigation, not only comes to a different conclusion, but actually obtains an admission of Adams's innocence from the original suspect of the case, David Harris. The "thin blue line" in the title "refers to what Mr. Morris feels is an ironic, mythical image of a protective policeman on the other side of anarchy".

The film won many awards, but was a controversial film among documentary film critics, who felt the use of reenactment had no place in the documentary format. For this reason, the film was not nominated for an Academy Award, though it won several other awards for best documentary. Roger Ebert "attributed its rejection to the inability of academy voters to appreciate innovative film making."

In 2001, the film was selected by the Library of Congress for preservation in the United States National Film Registry as being "culturally, historically, or aesthetically significant".