Ecumenism (), also spelled oecumenism, is the concept and principle that Christians who belong to different Christian denominations should work together to develop closer relationships among their churches and promote Christian unity. The adjective ecumenical is thus applied to any interdenominational initiative that encourages greater cooperation and union among Christian denominations and churches.

The fact that all Christians belonging to mainstream Christian denominations profess faith in Jesus as Lord and Saviour over a believer's life, believe that the Bible is the inspired word of God (John 1:1), and receive baptism according to the Trinitarian formula is seen as being a basis for ecumenism and its goal of Christian unity. Ecumenists cite John 17:20-23 as the biblical grounds of striving for church unity, in which Jesus prays that Christians "may all be one" in order "that the world may know" and believe the Gospel message.

In 1920, the Ecumenical Patriarch of the Eastern Orthodox Church, Germanus V of Constantinople, wrote a letter "addressed 'To all the Churches of Christ, wherever they may be', urging closer co-operation among separated Christians, and suggesting a 'League of Churches', parallel to the newly founded League of Nations". In 1937, Christian leaders from mainstream Christian churches resolved to establish the World Council of Churches, to work for the cause of Christian unity; it today includes churches from most major traditions of Christianity as full members, including the Assyrian Church of the East, the Old Catholic Church, the Oriental Orthodox Churches, the Lutheran World Federation, the Anglican Communion, the Baptist World Alliance, the Mennonite churches, the World Methodist Council, the Moravian Church, the Pentecostal churches and the World Communion of Reformed Churches, as well as almost all jurisdictions of the Eastern Orthodox Church; the Roman Catholic Church participates as an observer, sending delegates to official gatherings.

Many regional councils affiliated with the World Council of Churches, such as the Middle East Council of Churches, National Council of Churches in Australia and Christian Churches Together, work for the cause of Christian unity on the domestic level, with member denominations including churches from the Oriental Orthodox, Lutheran, Roman Catholic, Eastern Orthodox, Methodist, Anglican, and Reformed traditions, among others.

Each year, Christians observe the Week of Prayer for Christian Unity for the goal of ecumenism, which is coordinated by the World Council of Churches and adopted by many of its member churches.

The terms ecumenism and ecumenical come from the Greek οἰκουμένη (oikoumene), which means "the whole inhabited world", and was historically used with specific reference to the Roman Empire. The ecumenical vision comprises both the search for the visible unity of the Church (Ephesians 4:3) and the "whole inhabited earth" (Matthew 24:14) as the concern of all Christians. In Christianity, the qualification ecumenical was originally and still is used in terms such as "ecumenical council" and "Ecumenical Patriarch", in the meaning of pertaining to the totality of the larger Church (such as the Catholic Church or the Eastern Orthodox Church) rather than being restricted to one of its constituent local churches or dioceses. Used in this sense, the term carries no connotation of re-uniting the historically separated Christian denominations but presumes a unity of local congregations in a worldwide communion.