The Basilica of St. Stephen in the Round on the Caelian Hill (Italian: Basilica di Santo Stefano al Monte Celio, Latin: Basilica S. Stephani in Caelio Monte) is an ancient basilica and titular church in Rome, Italy. Commonly named Santo Stefano Rotondo, the church is Hungary's "national church" in Rome, dedicated to both Saint Stephen, the first Christian martyr, and Stephen I, the canonized first king of Hungary. The minor basilica is also the rectory church of the Pontifical Collegium Germanicum et Hungaricum.

Since 1985, the cardinal priest who holds the title of S. Stephano has been Friedrich Wetter.

The earliest church was consecrated by Pope Simplicius between 468 and 483. It was dedicated to the protomartyr Saint Stephen, whose body had been discovered a few decades before in the Holy Land, and brought to Rome. The church was the first in Rome to have a circular plan. Its architecture is unique in the Late Roman world. Santo Stefano was probably financed by the wealthy Valerii family whose estates covered large parts of the Caelian Hill. Their villa stood nearby, on the site of the present-day Hospital of San Giovanni Addolorata. Saint Melania the Elder, a member of the family, was a frequent pilgrim to Jerusalem and died there, so the family had connections to the Holy Land.

The church was originally commissioned by Pope Leo I (440-461), with the date confirmed by ancient coins and by dendrochronology, which places the wood used in the beams of the roof to around 455 AD, but was not consecrated until after his death. The original church had three concentric ambulatories flanked by 22 Ionic columns, surrounding the central circular space surmounted by a tambour that is 22 m (72 ft) high and 22 m wide). There were 22 windows in the tambour but most of them were walled up in the 15th-century restoration. The central ambulatory had a diameter of 42 m (138 ft), and the outer one a diameter of 66 m (217 ft). Four side chapels extended from the middle ambulatory to the outer ambulatory, forming a Greek cross.

The church was embellished by Pope John I and Pope Felix IV in the 6th century with mosaics and colored marble. It was restored in 1139–1143 by Pope Innocent II, who abandoned the outer ambulatory and three of the four side chapels. He also had three transverse arches added to support the dome, enclosed the columns of the central ambulatory with brick to form the new outer wall, and walled up 14 of the windows in the drum.

In the Middle Ages, Santo Stefano Rotondo was in the charge of the Canons Regular of the Lateran, but as time went on it fell into disrepair. In the middle of the 15th century, Flavio Biondo (Flavius Blondus) praised the marble columns, marble-covered walls, and cosmatesque works-of-art of the church, but he added that unfortunately "nowadays Santo Stefano Rotondo has no roof". Blondus claimed that the church was built on the remains of an ancient Temple of Faunus. Excavations in 1969 to 1975 revealed that the building was not converted from a pagan temple but was always a church, erected under Constantine I in the first half of the 4th century.

In 1454, Pope Nicholas V entrusted the ruined church to the Pauline Fathers, the only Catholic Order founded by Hungarians. This is the reason why Santo Stefano Rotondo later became the unofficial church of the Hungarians in Rome. The church was restored in the 1450s by Bernardo Rossellino, probably under the guidance of Leon Battista Alberti.

In 1579, the Hungarian Jesuits joined the Pauline Fathers. The Collegium Hungaricum, established by István Arator in 1579, was merged with the Collegium Germanicum in 1580, and became the Collegium Germanicum et Hungaricum, because very few Hungarian students were able to travel to Rome from the Turkish-occupied, Kingdom of Hungary.