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 both of them, reported New York Post online.
"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," a source said.
"Jessica's stylist, Estee Stanley, who is also Justin's partner in his home decor business HomeMint, has been helping to plan the wedding," it added.
The couple splashed out on a private plane to fly their guests, including 'Saturday Night Live' comedian Andy Samberg and music producer Timbaland, to Italy for their special day.
"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," the source said.
Celebrations are already underway for the couple's special day. Guests were spotted enjoying a low-key drinks reception near Naples followed by an impressive fireworks display.
Biel reportedly arrived in Italy after picking up her wedding dress from a designer showroom in Paris.