Archaeological officials said the barrier would be made of cast iron columns and placed at street level at a distance varying from about 5 to 16 meters (yards) from the walls of one of the eternal city's most visited Roman ruins.
In recent years, bits of mortar and fragments of the monument have fallen to the ground, particularly after rain storms.
Last July, an Italian newspaper reported that the Colosseum was about 40 cm (16 inches) lower on the south side than on the north, suggesting it was in danger.
But officials said there was no threat to the stability of the massive monument, famous for hosting bloody gladiator fights in the days of the Roman Empire.
One of Rome's most recognisable symbols, the Colosseum is about to undergo its first comprehensive restoration in 73 years.
The 25 million euro ($32.33 million) project will start next month and end in 2015.
The project, which had been delayed by three years of bureaucratic problems, will include the cleaning and restoration of the entire building, known in Roman times as the Flavian Amphitheatre.
It will be carried out in phases so that the Colosseum, which receives hundreds of thousands of visitors a year, will remain open. Only part of it will be covered by scaffolding at any one time.
