Full communion is a communion or relationship of full agreement among different Christian denominations or Christian individuals that share certain essential principles of Christian theology. Views vary among denominations on exactly what constitutes full communion, but typically when two or more denominations are in full communion it enables services and celebrations, such as the Eucharist, to be shared among congregants or clergy of any of them with the full approval of each.

Full communion is an ecclesiological term for an established relationship between Christian denominations that may be constituted by shared eucharist, doctrine, and ecclesiology. Different denominations emphasize different aspects or define the term differently.

Several Protestant denominations base their idea of full communion on the Augsburg Confession which says that "the true unity of the church" is present where "the gospel is rightly preached and sacraments rightly administered." They believe that full communion between two denominations is not a merger, but rather is when two denominations develop a relationship based on a mutual understanding, respect and recognition of baptism and sharing of the Lord's Supper. They may worship together, exchange clergy, and share commitments to evangelism and service. For example, groups recognized as being in full communion with the Evangelical Lutheran Church, on this basis, include the Presbyterian Church (USA), Reformed Church in America, United Church of Christ, the Episcopal Church (United States), the Moravian Church, and the United Methodist Church. These churches are not necessarily in full communion with each other, however; each denomination is free to develop its own relationships with other churches. For example, The Episcopal Church, in addition to being a member of the Anglican Communion, is in full communion with the Evangelical Lutheran Church in America, Moravian Church (Northern and Southern Provinces), Mar Thoma Syrian Church of India, Old Catholic Churches of the Union of Utrecht, Philippine Independent Church, and the Church of Sweden. They are not, currently, in full communion with the Presbyterian Church (USA), Reformed Church in America, or the United Church of Christ, though they are currently in dialogue with other churches; including the United Methodist Church, Presbyterian Church (USA), and Roman Catholic Church.

The Catholic Church makes a distinction between full and partial communion: where full communion exists, there is but the one Church; partial communion, on the other hand, exists where some elements of Christian faith are held in common, but complete unity on essentials is lacking. Accordingly, they see the church as in partial communion with Protestants and in much closer, but still incomplete, communion with Orthodox churches. It has expressed this distinction in documents such as Unitatis redintegratio, the Second Vatican Council's decree on ecumenism, which states: "quite large communities came to be separated from full communion with the Catholic Church. [...] Men who believe in Christ and have been truly baptized are in communion with the Catholic Church even though this communion is imperfect".

Nonetheless, the Second Vatican Council used the word "communion" in a sense other than communio in sacris when speaking of Christians separated from the Catholic Church. The Catechism of the Catholic Church, citing the Second Vatican Council and Pope Paul VI, states:

"The Church knows that she is joined in many ways to the baptized who are honoured by the name of Christian, but do not profess the Catholic faith in its entirety or have not preserved unity or communion under the successor of Peter" (Lumen gentium 15). Those "who believe in Christ and have been properly baptized are put in a certain, although imperfect, communion with the Catholic Church" (Unitatis redintegratio 3). With the Orthodox Churches, this communion is so profound that it lacks little to attain the fullness that would permit a common celebration of the Lord's Eucharist

Full communion thus involves completeness of "those bonds of communion – faith, sacraments and pastoral governance – that permit the Faithful to receive the life of grace within the Church."

In Catholicism, the "universal Church" means Catholicism itself, from the Greek adjective καθολικός (katholikos), meaning "universal". The term particular church denotes an ecclesiastical community headed by a bishop or equivalent, and this can include both local dioceses as well as autonomous (or sui juris) particular churches, which include other rites such as the Latin Church and the Eastern Catholic Churches.