The 'SexyBack' singer sparked rumours that the couple were engaged after being spotted browsing a jewellery store for a ring.
"Justin was really taking his time looking round the rings. He made the staff get a couple out of the cabinets so he could properly look at them," a source told Metro.
The pair, who have been together since 2007, are said to have chosen to marry in Italy after attending a friend's nuptials in the country last October.
"They both like the idea of marrying in Italy in the summer," the source added.
Timberlake has previously dated Cameron Diaz and Britney Spears.