Pulisic sent in a corner for Malick Thiaw to head the visitors into a 12th-minute lead before further goals from Ćlvaro Morata and Tijjani Reijnders ensured back-to-back wins for Milan.
"Amazing result for us," Pulisic told CBS Sports. "I mean to come here and play is of course not easy [because of] their history in the champions league.
"A Tough place to come and play so for us to put on a performance like that and win the game, obviously a really special night for us."
VinĆcius JĆŗnior had equalized from the penalty spot in the 23rd for Madrid after he'd been tripped in the box.
But Pulisic again played a role as Milan went back ahead. His pass into the box found Rafael LeĆ£o, whose shot was saved by Andriy Lunin only for Morata to pounce on the rebound to restore the visitors' lead against his former club.
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