Beckham was in the same position as Ronaldo in 2003 when he quit United to join Real and the England star admitted he found it tough to walk away from Old Trafford.
Ronaldo must decide whether to push United to sell him after making it clear he wants to move to the Spanish champions and Beckham is convinced the Portugal international has been struggling to make his choice.
"Playing for Manchester United was my dream but so was playing for Real. It was an incredibly difficult choice to make and I'm sure it is the same for Cristiano," Beckham told the Sun.
"When I left Manchester United the only other club I wanted to play for was Real Madrid.
"Living in Madrid and playing for Madrid - there's not many better clubs than that apart from Manchester United, which is equal. But to have those two choices is good for him."