This paper proposes a Software Defined Network (SDN) architecture for GeoBroadcast in VANETs. We have implemented a component to automatically manage the geographical location of Road Side Units (RSUs), which are used as a basis for our GeoBroadcast routing. GeoBroadcast in a vehicular network supports periodic broadcast messages from a source vehicle to the destination vehicles that are located in a specific geographical region. In existing Intelligent Transport Systems (ITS), the GeoBroadcast mechanism can be implemented using traditional IP networking. Typically, every periodic warning messages received at nearest RSU from the source must be routed to the control center in ITS, where it is redirected to every other RSUs, that are located in the destination geographical region for broadcasting. As a result, huge overhead in the control center is produced and higher network bandwidth is consumed. However, in our SDN based GeoBroadcast mechanism, the first warning message received by the source RSU is sent to the SDN controller as a packet-in message. The SDN controller will decode the packet-in message and use topological and geographical information to set up the routing paths to the destination RSUs, by installing appropriate flow entries on the corresponding RSUs and intermediate switches, for the following periodic warning messages that are to be broadcasted. As per our simulation with OpenNet a significant reduction by 84% in controller overhead, 60% in network bandwidth consumption, and 81% in latency are achieved.