The tomb in the south niche, beneath the mosaic of St. Lawrence, is especially imposing. It bears no Christian symbols and was probably a pagan tomb of a noble Roman. Legend assigns it to Galla Placidia but she was probably never buried here.

This small cruciform building was erected between ca. 425 and 450 as an ante-chapel of the church of Santa Croce (destroyed) and dedicated to St Lawrence. It was almost certainly not used as the tomb of Galla Placidia (died 450, daughter of the Roman Emperor Theodosius I). Its plain exterior of blind arcades and pilasters contrasts with the predominant blue, green and gold interior, which contains one of the best-preserved Early Christian mural mosaics. In the lunettes of the cross-arms are representations of the Good Shepherd, the Martyrdom of St Lawrence, harts at the fountain of life and doves drinking from vases. Whereas the first two mosaics are executed in the Hellenistic tradition and have an airy quality, the others and the striking star patterns that decorate the barrel vaults of each arm are two-dimensional in form. One of eight Ravenna sites on the UNESCO World Heritage List.