YMCA, sometimes regionally called the Y, is a worldwide youth organization based in Geneva, Switzerland, with more than 64 million beneficiaries in 120 countries. It was founded on 6 June 1844 by Sir George Williams in London, originally as the Young Men's Christian Association, and aims to put Christian principles into practice by developing a healthy "body, mind, and spirit."

From its inception, it grew rapidly and ultimately became a worldwide movement founded on the principles of muscular Christianity. Local YMCAs deliver projects and services focused on youth development through a wide variety of youth activities, including providing athletic facilities, holding classes for a wide variety of skills, promoting Christianity, and humanitarian work.

YMCA is a non-governmental federation, with each independent local YMCA affiliated with its national organization. The national organizations, in turn, are part of both an Area Alliance (Europe, Asia Pacific, the Middle East, Africa, Latin America and the Caribbean, the United States, and Canada) and the World Alliance of YMCAs (World YMCA). Consequently, all YMCAs are unique, while following certain shared aims, such as the Paris Basis.

Imitator organizations include the Young Women's Christian Association (YWCA), the Young Men's Hebrew Association (YMHA), and the Young Men's Buddhist Association (YMBA). YMCA is also the subject of Village People's 1978 song "Y.M.C.A."