Distributed denial-of-service (DDoS) attacks are becoming more frequent and powerful. Traditional edge defense solutions can no longer keep up, and in-network defense solutions are needed that involve multiple Internet Service Providers (ISPs) to collaboratively defend against the attacks. While collaborative defense solutions are technically more effective at stopping large-scale attacks, the incentives for ISPs to deploy these solutions remain unexplored. In this study, we develop a game theoretic model to capture the economic benefits and costs of deployment for ISPs competing for customers. Through large-scale simulations at the Internet level, we find that the majority of ISPs on the Internet have an economic incentive to participate in DDoS defense, driven by competition; and that the severity of DDoS attacks and the level of competition affect an ISP’s charge for filtering DDoS traffic for its customers.