Miles has been battling leukemia since he was 18 months old. He is now in remission after ending his treatments in June, and on Friday, he battled the likes of the Riddler, Penguin and Edward "E." Nigma.
"This has turned into a full-blown phenomenon," said San Francisco Police Chief Greg Suhr, who called Batkid into service to fight crime.
Wearing a Batman costume, Miles began his day by rescuing a woman from cable car tracks and caught the Riddler trying to rob a bank. Lou Seal, mascot for the San Francisco Giants, was kidnapped by the Penguin, but Batkid came to the rescue.
Miles traveled in a black Lamborghini with a Batman decal and a police escort worthy of a dignitary. Crowds followed him wherever he went, chanting "bat kid, bat kid," and by the time he reached Union Square, there were thousands cheering him on.
The effort even caught the attention of President Barack Obama, who posted a Vine saying, "Way to go Miles, way to save Gotham!"
According to KGO-TV, Miles had no idea what was in store for him. He thought he was in San Francisco just to get a Batman costume.
The Associated Press contributed to this report.