In which comic book did Batman first appear?

Batman first appeared in Detective Comics #27.

The issue was cover-dated May 1939 and introduced the character as “the Batman.” Writer Bill Finger and artist Bob Kane developed the hero, although the history of Batman’s creation also involves contributions from other collaborators and later disputes over credit.

Batman’s debut story, “The Case of the Chemical Syndicate,” presents him as a mysterious crime fighter investigating a murder. Unlike Superman, Batman had no superhuman powers; his abilities came from training, intelligence, courage, and technology.

Batman #1 did not appear until 1940. That first solo issue introduced or featured important characters and stories, including the early appearances of the Joker and Catwoman. Detective Comics itself began publication in 1937, two years before Batman’s debut.