We report a rare case of lens siderosis with an undetectable intraocular foreign body by imaging. An 8-year-old boy presented with diminution of vision in the left eye since 3 months. His parents gave a preceding uncertain history of a foreign body injury to his left eye 3 months ago while playing. Presenting visual acuity in the left eye was perception of hand movements. Slit-lamp examination revealed a total white cataract with brownish-pigmented spots on the anterior capsule of the lens, but no intraocular foreign body was found. There was also no evidence of an intraocular foreign body on ultrasonography. Patient underwent cataract extraction with intraocular lens implantation. During the operation, a small (2×1×1 mm in size) intralenticular foreign body of metal material was found and removed carefully with a magnet. The patient regained 20/30 vision after surgery.