"It was built into the wall, and had been recycled as a building material, as often happened in the medieval era. Extracted from the ground, it revealed itself in all its beauty.
"The face is refined and gracious, young and feminine. All of which makes us think this could be a depiction of Dionysos."
The marble head will be cleaned and eventually put on display.
"Rome continues to surprise us every day," said Virginia Raggi, the mayor of the capital. The head of the statue… is in excellent condition. It's a marvel."
The head dates to between the 1st century BC and the 2nd century AD, according to Claudio Parisi Presicce, director of Rome's archaeological museums.
"The hollow eyes, which were probably filled with glass or precious stones, date it to the first centuries of the Roman Empire," he said.
"The surface is not completely visible because we haven't yet given it a thorough clean. We think that there could still be traces of the original colour conserved in the band around the hair."
Dionysus was the son of Zeus and Semele, the mortal daughter of the king of Thebes.
He was known as the god of wine, winemaking and grape cultivation, as well as of fertility, theatre and religious ecstasy.
He was worshiped by the ancient Greeks as one of the 12 Olympians before being incorporated into the Roman pantheon of gods as Bacchus.