Demi Lovato has reportedly reignited her relationship with Wilmer Valderrama.

The 20-year-old X Factor US judge has enjoyed an on/off romance with Wilmer, 32, since 2010.

The Hispanic-American actor joined Demi at a private event in West Hollywood this week to celebrate the season 2 premiere of The X Factor, where the couple were seen getting cosy.

"Wilmer stayed by Demi's side all night," an onlooker told the New York Post. "He was very protective of her and had his legs straddled around her chair while they watched the show."

The pair were seen having dinner together earlier this month.

After striking up a romance in 2010, Demi and Wilmer briefly reconciled in 2011.

Their reunion came after Demi was released from a treatment centre for an eating disorder and self-injury.

Demi was recently linked to One Direction member Niall Horan after the pair were spotted eating together at a restaurant in Los Angeles.

However, Demi shot down rumours of a romance, confirming that "nothing's going on".