The Synod of Diamper (Udayamperoor Synod) (Malayalam: ഉദയംപേരൂർ സൂനഹദോസ്, romanized: Udayampērūṟ Sūnahadōs), held at Udayamperoor (known as Diamper in non-vernacular sources) in June 1599, was a diocesan synod, or council, that created rules and regulations for the ancient Saint Thomas Christians (also known as Mar Thoma Nasranis) of the Malabar Coast, a part of modern-day Kerala state, India, formally subjugating them and downgrading their whole Metropolitanate of India as the Diocese of Angamale, a suffragan see to the Archdiocese of Goa administered by Latin Church Padroado missionaries. This synod also introduced forced Liturgical Latinisation and the eschewal of local practices and beliefs, leading to a significant ecclesial protest by Saint Thomas Christians known as Coonan Cross Oath and a subsequent schism in the mid-17th century.

A synod is a council of a Christian denomination, usually convened to decide an issue of doctrine, administration or application. The word synod comes from the Ancient Greek σύνοδος 'assembly, meeting'; the term is analogous with the Latin word concilium 'council'. Originally, synods were meetings of bishops, and the word is still used in that sense in Catholicism, Oriental Orthodoxy and Eastern Orthodoxy. In modern usage, the word often refers to the governing body of a particular church, whether its members are meeting or not. It is also sometimes used to refer to a church that is governed by a synod.