"We have found traces of erotic decorations in blue, red and green," Rossella Rea, director of the 2000-year-old amphitheatre, said.
The fragments "seem to depict the glory of the gladiator world, with laurels, arrows, victory wreaths and even erotic scenes," the Repubblica newspaper said.
The frescoes were found in a corridor currently closed to the public while archaeologists were working to restore an area between the second and third floor of the Colosseum, which has fallen into disrepair in recent years.
"We have also found writing dating back to the 17th century as well as the signatures of spectators and foreign visitors" who had come to watch the Colosseum's famed gladiatorial contests and mock sea battles, Rea said.
"We hope to be able to find other traces in this corridor but that depends on the funds available to continue with the restoration," she added.
The frescoes are located in an area covering several square feet in a corridor which is around sixty metres long, and should be open to the public by summer 2014, Rea said.
The Colosseum, which was completed in 80 AD by the Roman emperor Titus and is now one of the most visited sites in the world, is in a pitiful state.
Bits of stone, blackened by pollution, have fallen off in previous years, and some experts have voiced concern that the foundations are sinking, giving the amphitheatre a lean.
The number of visitors to the Colosseum, which measures 188 metres by 156 metres and is 48.5 metres high, has increased from a million to around six million a year over the past decade thanks mainly to the blockbuster film Gladiator.