This paper focuses on defending against a novel attack targeting PoW-based blockchains—namely, the Selfish Denial-of-Service (SDoS) attack. SDoS requires as little as \(19.6\%\) of the total computational power to earn revenue exceeding its proportional share. Alarmingly, as the revenue-to-cost ratio of Bitcoin mining has recently approached 1, SDoS attacks can easily induce all rational honest miners to voluntarily cease mining altogether. To defend SDoS, this paper proposes a game-based strategy to defend against the SDoS attack. This method restricts the strategy whereby a node publishes only the block header after finding a new block and later reveals the full private chain immediately after a competing block is observed. Experimental results show that the proposed method not only reduces the attacker’s revenue, thereby encouraging voluntary termination of the attack, but also mitigates the impact of the SDoS attack and increases the revenue of honest miners.