The Romanian, Gheorghe Hagi, struck the vital equaliser with 13 minutes remaining after Bayern had fought back from an early goal to claim a 2-1 lead.
Oscar Garcia put the Spaniards ahead in the 15th minute but Bayern, looking for their first European trophy since 1976, fought back and Marcel Witeczek and Mehmet Scholl scored in the second half.
However, Hagi took advantage of a calamitous back pass from Markus Babbel, Bayern's German international defender, to fire home with his left foot in the 77th minute.
"We are going to have to play better than we did in the first half in front of 90,000 fans in Barcelona," Rehhagel said. "Barcelona played some great one-touch football."
Christophe Dugarry capitalised on an early mistake by the Slavia Prague defence to give Bordeaux a 1-0 away victory in the other semi-final.
Dugarry, who struck twice in Bordeaux's shock quarter-final victory over Milan, was unmarked when he scored in the seventh minute.
He came close to increasing Bordeaux's lead in the first half, hitting the bar and forcing a string of saves from the Slavia goalkeeper.Reuse content