Kingella kingae is a bacteria involved in developing arthritis in children. Its diagnosis remains difficult. We report a case for which a new biomarker, calprotectin measured in the synovial fluid, was strongly positive and a specific molecular test was the only way to diagnose it specifically. Kingella kingae remains as a common etiology of child arthritis. Its growth is sometime difficult. Even if synovial fluid (SF) culture in blood bottle or SF‐specific K. kingae PCR have improved the diagnosis, a new biomarker such as calprotectin may be useful for the diagnosis. [ABSTRACT FROM AUTHOR]