No one was injured when about 9 meters (30 feet) of the Pincio wall fell over above Piazza del Popolo before dawn on Wednesday. Rome Mayor Gianni Alemanno inspected the damage and promised the wall would be repaired in two weeks.
City culture officials blamed a winter of heavy rains and rare snowfall, followed by this summer's hot, dry weather for causing an outer layer of brickwork to detach.
Alemanno said the collapse could have been avoided with regular maintenance but that heavy budget cuts prevented that.
Pincio terrace near Villa Borghese park has long been a popular strolling place for Romans and tourists to admire the cityscape.