Only in the sense that he occasionally claimed to be; many witnesses attested to him eating liver, Bavarian sausages and stuffed squab (young pigeon). The latter was said to be his favorite dish by a restaurant that he attended regularly in the 1930s. According to Robert Payne (Hitler's biographer), the vegetarian claim was made up by Goebbels to give the impression of an ascete in the manner of Gandhi; Goebbels claimed that Hitler did not smoke, drink, eat meat or have anything to do with women. In fact, it is known he did all of these -- except smoke. So, apart from his many other faults, Hitler was also dishonest about his diet.
Hitler was actually not a vegetarian at all. That is a myth. Rynn Berry an educator and historian wrote a pamphlet about this and later expanded it into a book.
I read in a book that he indeed was a vegetarian, for a short time, but that was for a public relations campaign to display himself as "pure". Then he stopped, since it's impossible for a cold-blooded killer to make decisions to save lives if you know what I mean. Hitler's doctor recommended that he cut down on the amount of meat he ate, which he did to a small extent, Goebbels started the story that Hitler didn't smoke, drink, eat meat or have anything to do with women (it's well attested that Hitler did all of these apart from smoke).
Hitler was not a vegetarian. He has been recorded eating meat throughout his life and only occasionally refused meat at a meal because his doctor advised.