Dazai is a young man with mildly wavy, short, dark brown hair and narrow dark brown eyes. His bangs frame his face, while some are gathered at the center of his forehead. He is quite tall and slim in terms of physique.

For his attire, Dazai wears a long sand-colored trench coat, the belt of which he leaves untied. Under it is a black vest over a striped dress shirt that is light blue in hue. He wears a bolo tie, which is held by a brown ribbon and a turquoise pendant. He also dons beige pants, dark brown shoes, and has bandages wrapped around his entire body; only his face, hands, and feet are left uncovered. Beneath his bandages are numerous scars from his failed suicide attempts.

In BONES' anime adaptation, his blue dress shirt is simplified to a solid blue. This change is not present in promotional art and in the mobile game.

Back when he was in the mafia, Dazai wore a black trench coat that reached under his knees. He also wore bandages over his right eye and his right arm is in a sling—presumably products of his suicidal pastimes. At some point, he was able to regain the use of his right arm; the same arm he then used to aim and fire a gun at Akutagawa three times.