Background <p>There are several clinical signs that may contribute to the diagnosis of keratoconus. Their appearance is associated with the evolution of keratoconus. In this case, the formation of a halo on the iris in a patient with stage IV keratoconus was reported. This halo sigh is a novel one and has not been reported earlier.</p> Case presentation <p>A 30-year-old woman presented to the ophthalmology clinic with one year history of vision deterioration to finger counting in her left eye. A diagnosis of keratoconus was made due to the inferior-central corneal steepening and thinning. When the slit-lamp light projects onto the cone, a halo forms on the iris and shifts its shape and location as the irradiation angle changes.</p> Conclusion <p>A halo sign was observed on the iris of a patient with stage IV keratoconus (Amsler-Krumeich classification) when the slit-lamp beam was projected onto the corneal cone.</p>