"The Adventure of the Illustrious Client" (1924) is one of the 56 Sherlock Holmes short stories written by British author Sir Arthur Conan Doyle, and one of the 12 stories collected as The Case-Book of Sherlock Holmes (1927). It was first published in Collier's in the United States in November 1924, and in The Strand Magazine in the United Kingdom in two parts, in February and March 1925.

Sir James Damery comes to see Holmes and Watson about his illustrious client's problem (the client's identity is never revealed to the reader, although Watson finds out at the end of the story; it is heavily implied to be King Edward VII). General de Merville's young daughter Violet has fallen in love with the roguish and sadistic Austrian Baron Adelbert Gruner, who Damery and Holmes are convinced is a shameless philanderer and a murderer. His victim was his last wife, of whose murder he was acquitted owing to a legal technicality and a witness's untimely death. She met her end in the Splügen Pass. Holmes also finds out that the Baron has expensive tastes and is a collector and a recognised authority on Chinese pottery.

Holmes's first step is to see Gruner, who is amused to see Holmes trying to "play a hand with no cards in it". The Baron will not be moved and claims that his charm is more potent than even a post-hypnotic suggestion in conditioning Violet's mind to reject anything bad that might be said about him. Gruner tells the story of Le Brun, a French agent who was crippled for life after being beaten by thugs after making similar inquiries into the Baron's personal business.

Holmes gets some help with his mission in the form of Shinwell Johnson, a former criminal who now acts as an informer for Holmes in London's underworld. Johnson rakes up Miss Kitty Winter, the Baron's last mistress, who remains bitter at his treatment of her. She is bent on revenge and will do anything to help Holmes. Kitty tells Holmes that the Baron "collects women" and chronicles his conquests in a book. Holmes realises that this book, written in Gruner's own hand, is the key to curing Violet of her devotion to the scoundrel. Kitty tells Holmes that this book is kept in the Baron's study.

First, Holmes goes to see Violet, bringing Kitty with him, but Violet is proof against Holmes's words. Kitty then makes it clear that Violet might end up ruined, or murdered, by Gruner if she continues. Violet still stands firm, and the meeting ends with Holmes narrowly averting a public scene involving the enraged Kitty.

Later, Holmes is attacked by two of Gruner's men, and the newspapers imply that he is near death. Watson goes to 221B Baker Street, only to discover that Holmes's injuries have been exaggerated to give the impression that he will be out of action for quite a while. Several days later, Holmes is sufficiently recovered to be out of bed. The Baron is planning a trip to the United States just before the wedding, and will be leaving in three days. Holmes knows that Gruner will take his incriminating book with him, never daring to leave it behind in his study.

Holmes orders Watson to learn everything that he can about Chinese pottery in the next 24 hours. The next day, Holmes presents Watson with a fake business card styling him as "Dr. Hill Barton" and an actual piece of Ming pottery, a saucer. He is to go to Baron Gruner's house, pose as a connoisseur of Chinese pottery, and try to sell the saucer. Watson does as Holmes tells him, but cannot fool the Baron for very long, and Gruner soon realises who has sent him.

As Watson faces his murderous captor, a noise from another room alerts the Baron, and he rushes into his study just in time to see Holmes jump out of the window, having stolen the book. The Baron rushes to the window, but Kitty Winter, who has been hiding outside, throws oil of vitriol in his face. Watson applies treatment to his injuries until further assistance comes.