|Family||Leah MacGrady (grandmother)|
Martha (great aunt)
|Book debut||Arthur and the Pen-Pal Playoff|
Justin is Mrs. MacGrady's grandson. He is a third grader on the other side of the state from where Arthur lives. He likes playing basketball, and occasionally comes to Elwood City to visit his grandma. Mrs. MacGrady arranged for Arthur and Justin to be pen pals, that way they could meet each other.
Justin is a cat with a tan complexion. He wears a white shirt, blue shorts, white socks, and blue and white sneakers. He has brown hair.