The pair was spotted enjoying an off-Broadway performance of Sleep No More on Friday, fuelling rumours that they are becoming more than just friends.
A source claiming to be close to Ashley told People magazine, 'I'm not sure if Ashley is serious with Justin or not. They aren't dating but they have hooked up recently.
'Ashley is a relationship person, much more than [her sister, Mary-Kate], so we'll see.'
But a rep for Timberlake, 30, who split with Jessica Biel earlier this year after four years together, insisted, 'Justin is not romantically involved with anyone', while a second insider said the duo 'have mutual friends' and 'were hanging out as a group'.