John Patrick Ryan, Sr. KCVO (Hon.), nicknamed Jack, is a character created by American author Tom Clancy in 1984. He is the main character of the Ryanverse series of novels, films, video games, and a TV series. Clancy wrote fourteen novels featuring Ryan, which have charted at the top of the New York Times bestseller list.

A former U.S. Marine lieutenant and stockbroker, Ryan worked as a civilian history professor at the United States Naval Academy. He became an analyst working for the Central Intelligence Agency during the Cold War. He goes up the ranks at the CIA, eventually becoming Deputy Director. After a brief retirement, Ryan served as National Security Advisor and Vice President before becoming President of the United States following a terrorist attack at the United States Capitol. He went on to serve two non-consecutive terms and mostly dealt with international crises in Europe, South America, and Asia.

Since Clancy's death in 2013, there have been other authorized writers of novels featuring Ryan, including Mark Greaney, Marc Cameron, and Andrews and Wilson. Additionally, a spin-off series featuring Ryan's son Jack Junior was written by Grant Blackwood, Mike Maden, Don Bentley, and M. P. Woodward.

Ryan has been portrayed in adaptations by actors Alec Baldwin, Harrison Ford, Ben Affleck, Chris Pine, and John Krasinski. The film series from 1990 to 2014 have an unadjusted worldwide gross revenue of $788.4 million to date, making it the 57th-highest-grossing film series. A television series from Amazon Prime Video ran from 2018 to 2023.

Clancy developed the Jack Ryan character while working as an insurance agent in Maryland, drawing extensively from his early interests and personal background. Clancy, who grew up in an Irish-American family in Baltimore and was unable to serve in the military due to nearsightedness, developed a passion for military history and strategy from an early age. He was described as a "nerd" who enjoyed playing military board games and was an avid reader of military history books and science fiction.

The author's research methodology became central to his character and world-building process. Many of Clancy's insurance clients were former nuclear submariners, both officers and enlisted personnel, whose expertise he systematically gathered. He conducted extensive research using military publications such as Armed Forces Weekly and Jane's Defence Weekly, and collaborated with subject experts ranging from Soviet defectors to retired Air Force generals. Clancy based his first novel on the real-life attempted defection of the crew of the Soviet frigate Storozhevoy to Sweden.

According to character sketches discovered by the U.S. Naval Institute, Clancy originally envisioned Ryan as born in the 1950s, the son of a Baltimore police officer and hospital nurse. The character was designed to attend Clancy's own alma mater, Loyola High School, and earn an economics degree from Boston College before joining the United States Marine Corps. The detailed background included Ryan's career path from a helicopter crash injury during military service, through work at Merrill Lynch, to eventual recruitment by the CIA while working as an associate editor with the U.S. Naval Institute.

The character of Jack Ryan was modeled partly on former Director of Central Intelligence Robert Gates, with Clancy telling Gates that "You know, for the first several novels, I pretty much modeled Ryan's career on yours". This grounding in real-world intelligence careers helped establish the series' commitment to authenticity in depicting government operations and military technology.