Officials said the wall was part of a 2,000-year-old house on the Vicolo del Modesto, in a section of the Roman site that had already been declared off limits to the public for safety reasons.
About two square metres of the wall were involved in the collapse, which occurred after heavy rain storms in most of southern Italy.
Pompeii, which is a UNESCO World Heritage site, was frozen in time when Mount Vesuvius erupted in 79AD, with a force equivalent to 40 atomic bombs, burying 13,000 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.
The most serious was in 2010 when part of the 'House of the Gladiators', used for combat training, crumbled.
A ¤105m fund in Italian and European Union funds is already in place to restore the ancient city.
Pompeii attracts some 2.5 million tourists each year, making it one of Italy's most popular attractions.