Abstract: A smoke-detector security cover has a toroidal base plate attachable to a wall or ceiling surrounding a smoke detector. A toroidal cover plate having a plurality of retainer appendages extended inward radially is connected to the toroidal base plate to cover portions of the smoke detector circumferentially external from a central smoke-detection section of the smoke detector. Attachment rods are extended perpendicularly from the base plate towards the cover plate. Attachment sleeves are extended perpendicularly from the cover plate in sliding contact with the attachment rods telescopically. An anti-theft fastener means is provided in fastener relationship between at least one attachment rod and one attachment sleeve. The anti-theft fastener means can be a split attachment bolt that is tightened in an attachment sleeve with a tapered fastener head or tapered fastener shank. The fastener has an anti-theft head. The anti-theft head can be recessed to further deter unauthorized removal.