There are several games that you can say that the world's hardest sport. To my knowledge, Gymnastics is often called the world's hardest sport. It needs great strength, balance, and flexibility. Athletes train for years to master routines. Mistakes can cause injury. Both the mental and physical demands are very high. It combines dance, power, and perfect timing. Gymnasts start young and work hard every day. That’s why many say it's the hardest.