Biel, whose most recent movie was the "Total Recall" remake, is already in Italy for final dress fittings, E! said. The website also reported that friends and family have been signing nondisclosure agreements before flying in.
Us said the characteristically private couple have been taking precautions to keep the exact location of the small, rustic ceremony secret, but that celebrations with friends and family began Tuesday. It added that Biel was spotted at a bridal boutique Oct. 8, and that "Trouble with the Curve" and "Friends with Benefits" actor Timberlake had a bachelor's weekend in September that stretched from Las Vegas to Cabo San Lucas, Mexico.
Representatives for Timberlake, 31, and Biel, 30, did not respond to Newsday's requests for comment.
Biel sparked wedding rumors in April when she visited the Parisian salon of two major bridal designers, People magazine reported at the time.
Biel and Timberlake became engaged in late December after a romance that began in January 2007 but which suffered a three-month split in 2011, according to various media reports.
People and E! said the proposal took place in Montana, while Us specified Jackson, Wyo.