Wales top the group after their 2-1 victory over Slovakia at the weekend while England were held 1-1 by Russia in Marseille.
“It’s a big game,” Beckham said in a BBC interview.
“I think we will win but I don’t think it’s going to be easy. Chris Coleman has done such a great job and Wales have a great team spirit. It’s going to be a challenging game.”
Beckham, at London’ Queen’s Club for the traditional grasscourt warm-up ahead of the Wimbledon tennis championships, said England’s players would go into the match buoyant despite conceding a stoppage-time equaliser against Russia.
“We played well and the performance was great but the goal at the end was unfortunate,” said Beckham, who captained England at the 2002 and 2006 World Cups and 2004 European Championship.
“If we had scored the second goal, we would have raved about the performance. But they played with the passion the fans want to see and the energy and didn’t seem to care too much about being on the big stage. So that was good to see.”
England will be wary of Real Madrid’s Gareth Bale for a match that many predict will be like a Premier League tussle.
“It will be a great match to watch,” Beckham said. “Bale has had a great season and scored a great goal (against Slovakia).”