A decentralized cooperative guidance law for multiple missile groups in salvo attack is proposed by integrating the impact-time-control guidance (ITCG) with decentralized consensus algorithms. The missiles within a group communicate by means of the nearest-neighbor topology, while communication between groups is implemented through the root nodes of the group topology spanning tree. The decentralized coordinated strategies among different missiles and groups are provided for the cooperative guidance of multiple missile groups in salvo attack. A centralized optimal impact-time-control guidance law for multiple missiles is developed. The decentralized cooperative guidance law for multiple missile groups in salvo attack is designed based on the centralized optimal impact-time-control guidance law and the decentralized coordinated strategies. Finally the effectiveness of the proposed method is demonstrated by simulation.