The couple - who got engaged on their annual Christmas holiday last December after more than five years together - will exchange vows in a small town that holds special memories for them both.
A source told The New York Post's Page Six that the location ''means a lot to both of them''.
The source said: ''The wedding is happening outside of Naples in a tiny town this week. It is their favourite place, a location that is very special to them. There will be a few hundred guests, and the maid of honour will be Jessica's longtime best friend, Lindsay Ratowsky.
''Jessica's stylist, Estee Stanley, who is also Justin's partner in his home décor business HomeMint, has been helping to plan the wedding.''
The source said: ''A private plane was flown out of New York to take guests to Italy including both their publicists. But the event has been shrouded in a lot of secrecy, and not even the guests were told the exact location or day of the wedding before they left.''
Celebrations are already underway for the couple's special day. Guests were spotted enjoying a low-key drinks reception near Naples on Tuesday (16.10.12) followed by an impressive fireworks display last night (17.10.12).
The couple were first spotted visiting possible wedding locations in the Italian countryside last April, including Francis Ford Coppola's picturesque Palazzo Margherita in Bernalda, a small hilltop town 125 miles from Naples.
'Hitchcock' actress Jessica reportedly arrived in Italy on Tuesday after picking up her wedding dress from a designer showroom in Paris.