Officials said the wall was part of a 2,000-year-old house on the Vicolo del Modesto, in a section of the site that had already been declared off limits to the public for safety reasons.
About two square metres (yards) of the wall were involved in the collapse, which occurred after heavy rain storms in most of southern Italy.
The street where the collapse took place is located in an area of the dig that came to light in excavations in the 19th century.
Ancient Pompeii, which is a UNESCO World Heritage site, was frozen in time when Mount Vesuvius erupted in 79 AD, burying inhabitants alive and preserving their homes.
It has been the scene of a string of collapses in recent years, sparking international worry about the decay and perilous condition of the site. There were at least two serious collapses in 2011 and four in 2010.
The most serious was in 2010 when part of the "House of the Gladiators" - once used by gladiators to train before combat - crumbled.