The former BVB striker has followed the club's progress in Europe and is confident that they can beat their Bundesliga rivals at Wembley
The Bavarians hot favourites to claim the trophy for the fifth time in their history, having finished 25 points ahead of their rivals in the Bundesliga.
However, the former Switzerland star is confident that Jurgen Klopp's side can claim an unlikely victory at Wembley, having followed their progress throughout the season.
"Borussia Dortmund will win the Champions League!" he told Fifa.com.
"I knew BVB would get past Malaga [in the quarter-finals]. The pressure at Signal Iduna Park when the fans get behind the team is so enormous, only the truly great teams survive.
"Against Real Madrid [in the semi-final return leg], Dortmund could so easily have landed a lucky punch, as people call it, then it would have been basically all over from the start.
"But I do have to say if the game had gone on five or seven minutes longer, BVB would have been knocked out."
Frei went on to say that German teams are now setting the standard in European football.
"Bayern Munich and Borussia Dortmund have developed a playing style which could well set the benchmark for the next two years," he continued.
But an era like this comes along every two to four years. It's been Barcelona for a long time, but Chelsea signalled the end of the era last season, and this year it's been German teams.
"Having said that, the potential for German teams looks a lot rosier than for many others."