The 21-year-old joined Liverpool from Manchester United in 2013.
She helped the Reds win back-to-back Women's Super League titles in 2013 and 2014.
"I have been at Liverpool Ladies for a few years now, so I felt as though I needed a new challenge," Zelem told the club website.
"I have had a wonderful four years and will take away some great memories."
England Under-23 midfielder Katie Zelem has left Liverpool Ladies to join Italian side Juventus Women for an undisclosed fee.