The former England captain, who recently joined Paris Saint Germain, left Old Trafford for Real in 2003 and acknowledged his huge affection for both clubs in a Q & A session with H&M.
Beckham, who won six English titles, was unequivocal when asked whether the Red Devils would win this season's Premier League race ahead of rivals Man City - "Only one answer. Yes!"
The 37-year-old sees the meeting of two of his former clubs in a highly-anticipated Champions League last 16 tie as too close to call.
"Tough. I love both clubs," Beckham added. "I've been a United fan since I was five years old but am still in love with Real, it will be a great game."
While ruling out a move into management, Beckham did admit that buying into a club would be a possibility - but in response to a question he said that it would not be at the club where he played on loan at the start of his career.
"Management is not really in my thoughts to be honest," he added. "I have the option to buy a franchise in the MLS, so yes to the MLS, sadly a no to Preston."