is a 9th century minor basilica and parish and titular church, on ancient foundations. It is at Piazza San Marco 48, adjacent to the Piazza Venezia on the west hand side in the rione Pigna. The full name of the church is San Marco Evangelista in Campidoglio, (St Mark the Evangelist at the Capitol).
For reference, a plan of the church is available here
The basilica was probably founded by Pope St. Marcus (Mark) in 336 over an older oratory, and is one of Rome's oldest churches. It stands on the site where St Marcus is said to have lived, and was known as the Titulus Pallacinae. The church is thus recorded as Titulus Marci in the 499 synod of Pope Symmachus.
The church was rebuilt in the 5th century, and was left facing the opposite direction.
It was reconstructed in the 8th century by Pope Adrian I (772-795), and given back its original orientation, which it has today. It was flooded when the Tiber rose above its banks soon after, in 791. Pope Gregory IV (827-844) restored it after the flood, to which period belongs the mosaic in its tribune.
Pietro Cardinal Barbo, titular of the church and later Pope Paul II, ordered major changes in the architecture of the church in 1465-70, when the inside and the outside of the church were restyled according to the Renaissance taste. In that occasion the church was assigned to the Venetian people living in Rome, Paul II being a Venetian of birth. At the same time, he built the Palazzo Venezia, after the designs of Giuliano da Majano.
The last major rework of the basilica was started in 1654-57 and completed by Cardinal Angelo Maria Quirini in 1735-50. Card. Quirini erected the altar and tabernacle of the Blessed Sacrament and the great altar; and incrusted with Sicilian jasper the marble columns, which sustain the vaults of the aisles. With these restorations, the church received its current Baroque decoration by Filippo Barigioni.