Because nail psoriasis is difficult to treat, therapy with many biological drugs has been attempted. Ustekinumab is approved for chronic plaque psoriasis and psoriatic arthritis (PsA), with some trials reporting nail improvement using this agent. A 51-year-old man with severe chronic plaque psoriasis had severe involvement of all fingernails and toenails, with accompanying nail fold psoriasis. He also had PsA of the small joints of the fingers. Despite multiple conventional therapies, the nail lesions did not improve, and his nail psoriasis severity index score was 97. After a fourth ustekinumab injection, most of the fingernail psoriasis was resolved, and only hyperkeratosis remained on both large toenails. Because the nail plate, nail fold, and small joints of the fingers are closely apposed structures within a small area, cytokines produced from the nail units overflow to the nail fold and small joints and can induce nail fold psoriasis and PsA.