The Secret of the Unicorn (French: Le Secret de La Licorne) is the eleventh volume of The Adventures of Tintin, the comics series by Belgian cartoonist Hergé. The story was serialised daily in Le Soir, Belgium's leading francophone newspaper, from June 1942 to January 1943 amidst the Nazi German occupation of Belgium during World War II. The story revolves around young reporter Tintin, his dog Snowy, and his friend Captain Haddock, who discover a riddle left by Haddock's ancestor, the 17th century Sir Francis Haddock, which can lead them to the hidden treasure of the pirate Red Rackham. To unravel the riddle, Tintin and Haddock must obtain three identical models of Sir Francis's ship, the Unicorn, but they discover that criminals are also after three model ships and are willing to kill in order to obtain them.

The Secret of the Unicorn was a commercial success and was published in book form by Casterman shortly after its conclusion. Hergé concluded the arc begun in this story with Red Rackham's Treasure, while the series itself became a defining part of the Franco-Belgian comics tradition. The Secret of the Unicorn remained Hergé's favourite of his own works until creating Tintin in Tibet (1960). The story was adapted for the 1957 Belvision animated series Hergé's Adventures of Tintin, the 1991 Ellipse/Nelvana animated series The Adventures of Tintin, the 1992-3 BBC Radio 5 dramatisation of the Adventures, the feature film The Adventures of Tintin (2011) directed by Steven Spielberg, and the film's tie-in video game.

While browsing on the Brussels Voddenmarkt/Marché aux puces at the Vossenplein in the Marollen, Tintin purchases an antique model ship which he intends to give to his friend, Captain Haddock. Two strangers, model ship collector Ivan Ivanovitch Sakharine and antique-scout Barnaby, independently try and fail to persuade Tintin to sell the model to them. Tintin also meets detectives Thomson and Thompson on a secret mission looking out for a pickpocket; however, both their wallets are repeatedly stolen throughout the story. At Tintin's flat, Snowy accidentally knocks the model over and breaks its mainmast. Having shown the ship to Haddock, Tintin discovers that the ship is named the Unicorn, after a ship commanded by Haddock's own ancestor.

While Tintin is out, the ship is stolen from his apartment; in the following investigation, he discovers that Sakharine owns an identical model, also named the Unicorn. At home, Tintin discovers a miniature scroll, and realises that it must have been hidden inside the mast of the model. Written on the parchment is a riddle: "Three brothers joined. Three Unicorns in company sailing in the noonday sunne will speak. For 'tis from the light that light will dawn, and then shines forth the Eagle's cross". Captain Haddock vividly recounts the tale of the Unicorn, a 17th-century warship captained by his ancestor, Sir Francis Haddock, but seized by a pirate band led by Red Rackham. In a retold flashback, it is shown how Francis' ship is attacked by Red Rackham and his crew. At first the pirates raise the black Jolly Roger, indicating that quarter (safe conduct) is offered in exchange for peaceful surrender, but when Francis refuses to strike his flag and bombards the pirates with cannon fire, the pirates instead "hoists fresh colors" and raise the "red pennant" (a bloody flag, French: pavillon sans quartier), signaling no quarter (no mercy, no prisoners), after which they board Francis' ship and massacre his crew. The only one of his crew to survive the boarding, Sir Francis killed Red Rackham in single combat and scuttled the Unicorn; he later built three models, which he left to his sons.

Tintin deduces that each of the models contains a scroll, which united lead to the location of Red Rackham's treasure. Tintin and Haddock try to get the three scrolls, but they are out of luck: Tintin's wallet (with the first scroll) is stolen, while Sakharine is found drugged, and the scroll inside his own Unicorn missing. Meanwhile, Barnaby requests a meeting with Tintin, but is gunned down on Tintin's doorstep before he can speak, and points to sparrows as a cryptic clue to the identity of his assailant before falling unconscious. A few days later, Tintin is kidnapped and chloroformed by the perpetrators of the shooting: the Bird brothers, two unscrupulous antique dealers who own the third model of the Unicorn. They are behind the theft of Tintin's model and have also stolen Sakharine's parchment, knowing that only by possessing all three parchments can the location of Red Rackham's treasure be found.

Tintin escapes from the cellars of the Bird brothers' country estate, Marlinspike Hall, while the Captain arrives with Thomson and Thompson to arrest them. It is revealed that the Bird Brothers have only one of the parchments, as two were lost when their wallet was stolen. It is also revealed that Barnaby survived and has made a full recovery, much to Max Bird's enragement. The Bird brothers are arrested. Tintin and Thomson and Thompson track down the pickpocket, Aristides Silk, a kleptomaniac who has a penchant for collecting wallets, and obtain the Bird Brothers' wallet, containing the missing two parchments. By combining the three parchments and holding them to light, Tintin and Haddock discover the coordinates (20°37'42.0" N 70°52'15.0" W, 82 km north of the Dominican Republic) of the lost treasure and plan an expedition to find it.

The Secret of the Unicorn was serialized amidst the German occupation of Belgium during World War II. Hergé had accepted a position working for Le Soir, Belgium's largest Francophone daily newspaper. Confiscated from its original owners, the German authorities permitted Le Soir to reopen under the directorship of Belgian editor Raymond de Becker, although it remained firmly under Nazi control, supporting the German war effort and espousing anti-Semitism. After joining Le Soir on 15 October 1940, Hergé became editor of its new children's supplement Le Soir Jeunesse, with assistance by old friend Paul Jamin and cartoonist Jacques Van Melkebeke, before paper shortages forced The Adventures of Tintin to be serialised daily in the main pages of Le Soir. Some Belgians were upset that Hergé was willing to work for a newspaper controlled by the occupying Nazi administration, although he was heavily enticed by the size of Le Soir's readership, which reached 600,000. Faced with the reality of Nazi oversight, Hergé abandoned the overt political themes that had pervaded much of his earlier work, instead adopting a policy of neutrality. Without the need to satirise political types, entertainment producer and author Harry Thompson observed that "Hergé was now concentrating more on plot and on developing a new style of character comedy. The public reacted positively".

The Secret of the Unicorn was the first of The Adventures of Tintin which Hergé had collaborated on with Van Melkebeke to a significant degree; biographer Benoît Peeters suggested that Van Melkebeke should rightly be considered the story's "co-scriptwriter". It was Hergé's discussions with Van Melkebeke that led him to craft a more complex story than he had in prior Adventures. Van Melkebeke had been strongly influenced by the adventure novels of writers like Jules Verne and Paul d'Ivoi, and this influence is apparent throughout the story. The inclusion of three hidden scrolls has parallels with Verne's 1867 story, The Children of Captain Grant, which Van Melkebeke had recommended to Hergé. Hergé acknowledged Van Melkebeke's contribution by including a cameo of him within the market scene at the start of the story; this was particularly apt as Van Melkebeke had purchased his books in Brussels' Old Market as a child.