Seamus is sorted into Gryffindor. At dinner, he tells his housemates that he has a witch mother and a Muggle father. He tells them that his mother didn't tell his father she was a witch until after they were married, which was a nasty shock for him. Seamus is intrigued when Ron recognises Nearly Headless Nick, and wants to know how the ghost can be 'nearly headless'. He also asks how the Bloody Baron, the Slytherin ghost, came to be covered in blood.
Seamus shares classes with the other Gryffindors and is in the History of Magic class when Hermione asks Professor Binns about the Chamber of Secrets. Like the rest of the class, he becomes much more interested in what Binns says about this than he normally would during a History of Magic class.
Like the rest of the Gryffindors, Seamus becomes much more interested in the Divination class when Professor Trelawney announces that she sees the Grim in Harry's teacup, although he states that, with his head tilted one way, it looks more like a donkey.
In the Defense Against the Dark Arts lesson, Professor Lupin calls Seamus forward to face the Boggart, which turns into a wailing banshee. As he casts the charm to repel it, the Boggart banshee clutches at its throat, and loses her voice.
Seamus visits Harry, Ron and Hermione in their compartment on the Hogwarts Express and is still wearing his Ireland rosette, from the Quidditch World Cup. Some of the magic appears to have worn off, and it is still squeaking 'Troy! Mullet! Moran!', but in a very feeble and exhausted way.
Seamus joins his fellow Gryffindor students in greenhouse three for their Herbology lesson. He sounds revolted when Professor Sprout tells them they will be collecting Bubotuber pus during their lesson.