What was Adolf Hitler's middle name?

Adolf Hitler did not have a middle name. He was listed in the baptismal registry simply as Adolfus Hitler.

http://www.historyplace.com/specials/faq/index.html

He didn't have one.

This is interesting:
Hitler was not sure who his paternal grandfather was, but it was probably either Johann Georg Hiedler or his brother Johann Nepomuk Hiedler. There have been rumours that Hitler was one-quarter Jewish [2] and that his paternal grandmother, Maria Schicklgruber, had become pregnant after working as a servant in a Jewish household in Graz. During the 1920s, the implications of these rumours along with his known family history were politically explosive, especially for the proponent of a racist ideology. Opponents tried to prove that Hitler, the leader of the anti-Semitic Nazi Party, had Jewish or Czech ancestors. Although these rumours were never confirmed, for Hitler they were reason enough to conceal his origins. Soviet propaganda insisted Hitler was a Jew, though more modern research tends to diminish the probability that he had Jewish ancestors.
http://www.newsnfo.co.uk/pages/Adolf%20Hitler.htm

He didn't have one.

Here is a website you can look at it.

https://en.wikipedia.org/wiki/Adolf_Hitler

If one of his possible grandfathers had the names Johann Nepomuk then there is very likely to be a Czech connection. It's a very Czech name, Nepomuk - and almost always associated with Jan or Johann in the Germanised version. https://en.wikipedia.org/wiki/John_Nepomucene