The anonymous, typed letter was sent from Turin to the Islamic centre in Imperia, Liguria, Il Secolo XIX reported.
“Go back to your pigsty or you’ll all burn in the mosques,” the letter said.
The special branch of the Italian police are investigating the death threat, Il Secolo reported.
Roberto Hamza Piccardo, head of Imperia’s Islamic community, said he was not worried by the letter.
“There are 60 million Italian citizens. There will be some idiots too. But as the saying goes: do no evil, fear no evil. We are calm and continue to go forward,” he was quoted as saying.
Imperia’s Islamic community is made up of 3,000 members, including both Italians and foreign residents, the newspaper said.
Threats to Muslim places of worship have been made elsewhere in Europe, with police being called in to protect a mosque in Denmark. Last week swastikas were sprayed across a mosque in Vienna, the latest in a string of attacks against places of worship in the Austrian capital.
Mosques in France came under attack by shots and grenades last month, following deadly shootings in Paris by Islamic extremists.