An iridium (Ir) modified silicon (001) (Si(001)) surface is studied using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The surface exhibits p(2 × 2) domains in LEED intensity images. The STM images show that the basis of the crystal lattice consists of an Ir atom and a Si dimer and, like Si(001) dimer rows, they are aligned parallel to the orthogonal [110] directions.