Michael Hammer is a fictional character created by the American author Mickey Spillane. Hammer debuted in the 1947 book I, the Jury, and appeared in 13 novels by Spillane (the final published 1996), and 15 more published by Max Allan Collins after Spillane's death and based on Spillane's notes or incomplete manuscripts.

Spillane's Mike Hammer books were immensely successful in the 1950s, and Spillane's books rank among the best-selling fiction in modern publishing history with estimated sales over 100 million to 200 million. The Hammer character was also adapted to film and television on multiple occasions between the 1950s and early 2000s.

Hammer is a no-holds-barred private investigator whose love for his secretary Velda is outweighed only by his willingness to kill a killer. Hammer's best friend is Pat Chambers, Captain of NYPD Homicide. Hammer was a World War II army veteran who spent two years fighting jungle warfare in the Pacific Ocean theater of World War II against Japan.

In 1942, when he was a comic book writer, Spillane created the private detective Mike Lancer published in Green Hornet Comics #10 (December, 1942) by Harvey Comics. The character was partly based on Spillane's friend Jack Stang, an ex-U.S. Marine turned police officer in Newburgh, New York.

In 1946, Spillane worked with illustrator Mike Roy and Edwin Robbins to create the private-eye character Mike Danger for proposed comic-book or comic-strip publication. Unable to sell the project as a comic, he reworked the story as the novel I, the Jury, converting Mike Danger to Mike Hammer and supporting character Holly to Velda. "Mike Hammer originally started out to be a comic book. I was gonna have a Mike Danger comic book," Spillane said in a 1984 interview. Two Mike Danger comic-book stories were published in 1954 without Spillane's knowledge, as well as one featuring Mike Lancer (1942). These were published with other material in "Byline: Mickey Spillane," edited by Max Allan Collins and Lynn F. Myers, Jr. (Crippen & Landru publishers, 2004).

While pulp detectives such as Sam Spade and Philip Marlowe are hard-boiled and cynical, Hammer is in many ways the archetypal "hard man": brutally violent, and fueled by a genuine rage against violent crime that never afflicts Raymond Chandler's or Dashiell Hammett's heroes. In The Big Kill, Hammer describes himself to a bargirl as a misanthrope. Spillane admitted to pulp writer Carroll John Daly, generally regarded as the inventor of the hard-boiled private eye figure, that Hammer was also loosely modeled on Race Williams, Daly's most frequently used detective character.

While other hardboiled heroes bend and manipulate the law, Hammer often views it as an impediment to justice, the one virtue he holds in absolute esteem. Hammer nevertheless has a strong respect for the majority of police, realizing they have a difficult job and their hands are frequently tied by the law when trying to stop criminals.

Hammer is also patriotic and anti-communist. The novels are peppered with remarks by Hammer supporting American troops in Korea, and in Survival...Zero Vietnam. In One Lonely Night, where Hammer attends a communist meeting in a park, his reaction to the speaker's propaganda is a sarcastic "Yeah."