A microwave frequency divider based on an optically injected semiconductor laser is presented. It has a very simple structure as it only requires an optical circulator and a DFB laser in a conventional microwave photonic link. Period-two oscillation in the DFB laser, introduced by injecting an RF modulated optical signal into the DFB laser, generates optical frequency components with half of the input RF signal frequency separation. Beating of these optical frequency components at the photodetector produces a divide-by-two frequency component. The proposed microwave frequency divider is free of electrical components. It can operate over a wide input RF signal frequency range without the need of altering the system physical configuration. Experimental results demonstrate generation of a 1/2 frequency component for different input RF signal frequencies, by adjusting the forward bias current of a DFB laser.