The entirety of the church's interior is covered with 14th-century frescoes, which are described as "particularly remarkable" in a UNESCO publication. Among the notable mural portraits is that of church patron Saint Peter, whose larger-than-life image adorns the wall next to the iconostasis. Other frescoes include that of Jesus Christ painted to the right of the apse and that of Saint Alexius next to the entrance. The middle reaches of the walls feature images from the Holy Week, with frescoes depicting events from other major holidays above them. The conch of the apse boasts an image of the Mother of God; portraits of various saints and scenes from the Ascension of Jesus and the Annunciation complete the decoration of the apse.
