We aimed to present a novel case of sectoral ciliary body agenesis and complicated cataract as an embryogenic defect of eye development diagnosed by ultrasound biomicroscopy. A 20-year-old male patient presented with a complaint of visual impairment in his left eye since childhood. Slit-lamp examination of the left eye revealed pigment precipitation and focal lens opacities extending from the temporal quadrant through the posterior lens capsule, blocking the central optical axis. On ultrasound biomicroscopy examination, there was a hyperechoic reflection belonging to the rudimentary ciliary body structures between 2-5 o’clock in the temporal quadrant. The zonules could not be visualized in the same location. At all other quadrants of the anterior chamber angle, the ciliary body and zonules were normal. This is a very rare case of sectoral ciliary body agenesis complicated by cataract. Ultrasound biomicroscopy may be useful for detecting rare congenital anomalies of the anterior segment, anterior chamber angle, and ciliary body.