Santi Nereo e Achilleo is a Catholic church in Rome, in the Celio district, near the Baths of Caracalla. Built in the 4th century, it is dedicated to the martyrs Saints Nereus and Achilleus. In 814, Pope Leo III rebuilt the church to house the relics of the martyrs. In 1475 and 1600, it underwent significant restorations. The facade, restored by Pope Sixtus IV, features geometric frescoes by Girolamo Massei. The turrets date back to the time of Pope Leo III, and the Baroque bell tower was added in the 17th century during the restorations. The apse shows traces of bricked-up windows, and a 9th-century mosaic was restored in the 19th century.