The narthex of a church is the entrance or lobby area, located at the end of the nave, at the far end from the church's main altar. Traditionally the narthex was a part of the church building, but was not considered part of the church proper. It is either an indoor area separated from the nave by a screen or rail, or an external structure such as a porch.

The word comes from narthex (Medieval Latin from Classical Greek narthex νάρθηξ 1. giant fennel, 2. scourge) and was the place for penitents. In modern Greek narthekas (νάρθηκας) no longer has this meaning and is either the porch of a church, as English, or the brace of a sprained wrist or sling of a broken arm. In English the narthex is now the porch outside the church at the west end, formerly it was a part of the church itself.

The purpose of the narthex was to allow those not eligible for admittance into the general congregation (particularly catechumens and penitents) to hear and partake in the service. The narthex would often include a baptismal font so that infants or adults could be baptized there before entering the nave, and to remind other believers of their baptisms as they gathered to worship. The narthex is thus traditionally a place of penitence, and in Eastern Christianity some penitential services, such as the Little Hours during Holy Week are celebrated there, rather than in the main body of the church. In the Russian Orthodox Church funerals are traditionally held in the narthex.