Reactive oxygen species (ROS), which are key cellular signalling molecules, are reactive chemicals containing oxygen. Cell survival or death is a critical issue in the inflammatory response. Accumulation of ROS involves ROS generation and scavenging, which determine ROS homeostasis. Understanding the roles of ROS in modulating the inflammatory response in fish is vital for helping protect fish from the damage of water pollutants in harsh environmental conditions. ROS‐related key genes and signalling pathways are relatively conserved in fish but vary among different species. Recent frequent incidences of fish diseases have posed a considerable challenge to large‐scale aquaculture. ROS is important in stress perception, integration of diverse stress‐responsive signalling networks, and activation of animal defence mechanisms, which frequently occur during inflammation in fish. This review summarises recent studies on ROS signalling pathways during inflammation in fish. Furthermore, it examines the relationship between ROS and inflammation in fish. This review may contribute to the understanding of the underlying mechanisms by which ROS regulate inflammation in fish and provide suggestions for sustainable development in aquaculture.