It was the main church (Katholikon) of the monastery, dedicated to Our Lady. It was built in mid-14th century, probably by the first governor of Mystras, Manuel Kantakouzenos and his wife Isabelle de Lusignan. It is a domed, cross-in-square church with chapels. The lodge was replaced by a side narthex. The chapel of Saint Catherine on the west side, as well as the two chapels on the east side of the church, were built later. The church adjoins the natural rock of Mystras, hence its uneven architectural lay out.
The wall paintings of the church are considered to be the most significant and unique (the only preserved) examples of the late byzantine painting. These masterpieces were painted by four different painters and they date back to the third quarter of the 14th century.
Among the sculptures of the church, presently preserved, are the iconostasis, the arch icon stand and the embossed column capitals adorned with plant and geometrical motifs. In the interior of the church, visitors can see heraldic shields of the Kantakouzenos and de Lusignan dynasties.