Abstract: An anti-theft window guard comprising laterally-spaced hollow vertical posts secured at their respective opposite ends to a window frame by holder elements. The latter are formed with a flat seating flange and at least one inwardly-projecting dowel pin adapted to be removably fixed in the posts. At least two vertically-spaced horizontal rod members are adapted to extend through the vertical posts. In one embodiment, the grill structure defined by the posts and rod members is hingedly secured to the upper rail of the window frame and a locking device at the lower rail is provided. All embodiments are made of separate elements which can be easily cut to size, assembled and installed.