Kawasaki disease (KD) is a systemic vasculitis and is the most commonly acquired heart disease among children in many countries, which was first reported 50 years ago in Japan. The 2019 coronavirus disease (COVID-19, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)) has been a pandemic in most of the world since 2020, and since late 2019 in China. Kawasaki-like disease caused by COVID-19 shares some symptoms with KD, referred to as multisystem inflammatory syndrome in children, and has been reported in the United States, Italy, France, England, and other areas of Europe, with an almost 6-10 times or more increase compared with previous years of KD prevalence. Hydrogen gas is a stable and efficient antioxidant, which has a positive effect on oxidative damage, inflammation, cell apoptosis, and abnormal blood vessel inflammation. This review reports the chemical and biochemical aspects of hydrogen gas inhalation in treating KD and COVID-19.