Thunderball is a 1965 spy film and the fourth in the James Bond series produced by Eon Productions, starring Sean Connery as the fictional MI6 agent James Bond. It is an adaptation of the 1961 novel of the same name by Ian Fleming, which in turn was based on an original screenplay by Jack Whittingham devised from a story conceived by Kevin McClory, Whittingham, and Fleming. It was the third and final Bond film to be directed by Terence Young, with its screenplay by Richard Maibaum and John Hopkins.

The film follows Bond's mission to find two NATO atomic bombs stolen by SPECTRE, which holds the world ransom to the tune of £100 million in diamonds under threat of destroying an unspecified metropolis in either the United Kingdom or the United States (later revealed to be Miami). The search leads Bond to the Bahamas, where he encounters Emilio Largo, the card-playing, eyepatch-wearing SPECTRE Number Two. Backed by CIA agent Felix Leiter and Largo's mistress, Domino Derval, Bond's search culminates in an underwater battle with Largo's henchmen. The film's complex production comprised four different units, and about a quarter of the film comprises underwater scenes. Thunderball was the first Bond film shot in widescreen Panavision and the first to have a running time of over two hours.

Although planned by Bond film series producers Albert R. Broccoli and Harry Saltzman as the first entry in the franchise, Thunderball was associated with a legal dispute in 1961 when former Fleming collaborators McClory and Whittingham sued him shortly after the 1961 publication of the novel, claiming he based it upon the screenplay the trio had written for a cinematic translation of James Bond. The lawsuit was settled out of court and Broccoli and Saltzman, fearing a rival McClory film, allowed him to retain certain screen rights to the novel's plot and characters, and for McClory to receive sole producer credit on this film; Broccoli and Saltzman instead served as executive producers.

The film was exceptionally successful: its worldwide box-office receipts of $141.2 million (equivalent to $1,408,900,000 in 2024) exceeded not only that of each of its predecessors but that of every one of the next five Bond films that followed it. Thunderball remains the most financially successful film of the series in North America when adjusted for ticket price inflation. In 1966, John Stears won the Academy Award for Best Visual Effects and BAFTA nominated production designer Ken Adam for an award. Some critics and viewers praised the film and branded it a welcome addition to the series, while others found the aquatic action repetitious. The movie was followed by 1967's You Only Live Twice. In 1983, Warner Bros. released a second film adaptation of the Thunderball novel under the title Never Say Never Again, with McClory as executive producer.