The popstar admitted himself to The Priory after being given an ultimatum by his fiancée Emma Griffiths, according to The Mirror.
Griffiths apparently demanded that her partner got help after he spent a night partying with Amy Winehouse last month.
"He went too far after the night out with Amy," said a source. "He was so out of it he started swinging at photographers.
"He and Emma are supposed to get married next month and have a massive magazine deal but she told him he needs to sort himself out before she becomes his wife.
"She made him promise that he would give up drugs and heavy drinking - and she's devastated he broke that promise."
Willis has visited rehab in 2005 and 2006 for alcohol and drug addictions.