The 20-year-old Portugal international, who earns an estimated £50,000 per week playing for United, went voluntarily to a police station in London yesterday to be questioned about the allegations. His alleged victim claims she was sexually assaulted in a penthouse suite at the Sanderson Hotel in London on 2 October, after United beat Fulham.
After police interviews with Mr Ronaldo and the alleged victim, the Crown Prosecution Service will decide whether to lay charges.
A second man in his 30s also kept an appointment at the police station. He is thought to have been with the player on the night but has not been arrested. The claims against Mr Ronaldo are thought to have been made by two women and resulted, when they were first made, in speculation about the Premiership player's identity.
Mr Ronaldo was transferred to Manchester United from Sporting Lisbon for £12.3m in 2003. The club made no statement about his arrest.
The case follows a series of high-profile sex allegations against footballers. In September last year two Premiership players were interviewed over an alleged gang rape at the Grosvenor House hotel in London. No charges were laid.
Also last year, three Leicester City players faced allegations of sexually assaulting three women in a five-star hotel on a training camp in Spain. They spent six nights in jail but all charges were dropped.
In January last year, rape charges were dropped against the ex-Chelsea player Jody Morris and a friend after they were accused of raping a woman in Leeds.
Arsenal's Dutch striker Robin van Persie was held in jail in Rotterdam earlier this year after being accused of rape. He was released after two weeks and maintains his innocence.