Who is marvels first superhero?

The first character to appear in a comic book that would later be part of the Marvel Universe was Prince Namor the Sub-Mariner, in a title called 'Motion Picture Funnies Weekly' from April of 1939; this story was expanded for the first issue(#1) of the magazine titled 'Marvel Comics' in Oct/Nov 1939.

'Marvel Comics #1' was the first publication by Timely Publications, and also featured stories with the original Human Torch, Ka-Zar, the original Angel, and a few others.

However, the first comic that was published under the *logo*(or brand-name/company imprint, etc) of "Marvel Comics" was 'Amazing Adventures #3' in August 1961, and featured the man who would later be known as Dr. Druid(here known as Dr. Droom, and trained by the same sorcerer as Dr. Strange would be later).

Spider-Man would debut later in Amazing Adventures #15(after it had changed names to Amazing Fantasy) in August of 1962, and the Fantastic Four would appear in their own title in November of 1961.

Dr. Druid, however, debuted in Amazing Adventures #1 and continued through the title until it was the first official Marvel Comics publication with issue #3, beating the Fantastic Four by three whole months(Aug to Nov of 1961)...so it can be argued quite conclusively that Dr. Druid is the first *official* Marvel superhero, from a lot of different angles!

This is, of course, disregarding any *in-story* histories that establish Captain America or Nick Fury or King Arthur or Wolverine or the Olympian Gods or the Eternals, etc, etc, etc, as characters or super-heroes who existed *prior* to the publication of these issues in "our world".