Pyogenic arthritis with gas gangrene triggered by intra-articular steroid injections can occasionally result in fatal complications. Clostridium perfringens is typically the causative pathogen, with infections caused by Fusobacterium sp. being relatively rare. Fusobacterium sp. are known to cause pyogenic infections, but due to their extreme sensitivity to oxygen, they can be difficult to detect with traditional culture methods. Recently, metagenomic next-generation sequencing (mNGS) has gained attention as an alternative diagnostic tool to traditional culture, enabling rapid identification of causative pathogens in infectious diseases, including pyogenic arthritis. Its use is illustrated in the following case report, which demonstrates the diagnostic utility of mNGS in pyogenic arthritis with gas gangrene triggered by an intra-articular steroid injection. Using mNGS as a complement to conventional culture testing allows for a more precise narrowing-down of causative pathogens, enabling targeted therapy and improving patient outcomes. This approach may also help reduce the use of broad-spectrum antibiotics and prevent the emergence of antibiotic-resistant bacteria.