Armando Dippet served as Hogwarts headmaster immediately before Albus Dumbledore in J.K. Rowling’s books.
Armando Dippet appears in the history of Hogwarts as the headmaster who preceded Dumbledore. In Harry Potter and the Chamber of Secrets, his office portrait and memories help establish the earlier period when Tom Riddle was a student at Hogwarts. Dippet was headmaster when the Chamber of Secrets was opened and a student named Myrtle Warren was killed.
Dumbledore later became headmaster and remained one of Hogwarts’ defining leaders for much of Harry’s education. Because Dumbledore dominates the school’s modern history in the novels, readers may overlook Dippet’s earlier tenure.
Dippet should not be confused with Phineas Nigellus Black, who was another former headmaster, or with Severus Snape, who became headmaster much later. The question concerns the immediate predecessor to Dumbledore, not every headmaster who held the position before him.