Abstract: A tape measure marker device is attachable to a common tape measure blade. The marker device has a central body portion with a pair of downwardly depending legs, each with an inwardly facing flange that helps hold the marker device to the tape measure blade. The ends of the facing flanges are spaced apart and together with the underside of the two legs and the central body portion form an open cavity that is configured to receive a common tape measure blade. The convex underside of the central body portion is adapted to nest with the top face of the tape measure blade. In operation, the marker device is manually slid along the tape measure blade until a registration mark, which may be printed, embossed or engraved on the surface of the device, aligns with a point that is to be measured. The coplanar underside of the flanges on the device allows the marker device and the tape measure blade to lie directly against a device that is to be measured.