Final answer:
Severus Snape was the Defense Against the Dark Arts teacher in Harry Potter's seventh year at Hogwarts.
Step-by-step explanation:
In Harry Potter's seventh year at Hogwarts, the Defense Against the Dark Arts teacher was Severus Snape. Throughout the series, Snape played a complex role in the story, serving as both a Potions professor and later as the Defense Against the Dark Arts teacher. However, it is important to note that the position of Defense Against the Dark Arts teacher changed every year in the Harry Potter series, except for in Snape's case.