John M. "Jack" Keane (born February 1, 1943, in New York City) is a retired United States Army four-star general and former Vice Chief of Staff of the Army (1999–2003). A graduate of Western Kentucky University (B.S. in accounting, 1966) who later earned an M.B.A. from Western New England University (1976), Keane was commissioned as a second lieutenant in 1966 and served a 37-year military career that included two tours in Vietnam—where he commanded an airborne ranger company and a battalion—and command of the 101st Airborne Division during Operation Desert Storm in 1991. He rose to become the Army’s second-highest-ranking officer as Vice Chief of Staff.

Keane has been an influential voice on military transformation and defense policy, advocating increased defense spending and changes to U.S. force structure, particularly after 9/11. He played a key role in shaping U.S. strategy in Iraq and Afghanistan, was a vocal supporter of the 2007 Iraq "surge," and has frequently criticized perceived weaknesses in U.S. military readiness. He has collaborated with analysts such as Frederick Kagan and contributed to policy work associated with think tanks and the Institute for the Study of War; he serves as chairman of the Institute for the Study of War. Keane is known for his strong pro-Israel views.

Since retiring in 2003, Keane has remained active as a national security commentator and Fox News military analyst, a senior advisor to defense contractors (including Academi, formerly Blackwater), and a director or board member of several firms and nonprofits—listed affiliations have included American International Group (AIG), the Combating Terrorism Center at West Point, and other defense-related organizations. His ties across military, corporate, and media sectors have made him a prominent voice in public and policy discourse on national security.