The purpose of this systematic review was to describe the characteristics of clinical trials that focused on COVID-19 patients with cytokine release syndrome (CRS) and the variability in CRS definitions. Two authors independently searched three clinical trial registries and included interventional clinical trials on COVID-19 hospitalized patients that required at least one elevated inflammatory biomarker. Relevant data, including the type and cutoff of the measured biomarker, oxygen/respiratory criteria, fever, radiologic criteria, and medications, were summarized. A total of 47 clinical trials were included. The included studies considered the following criteria: oxygen/respiratory criteria in 42 trials (89%), radiologic criteria in 29 trials (62%), and fever in 6 trials (18%). Serum ferritin was measured in 35 trials (74%), CRP in 34 trials (72%), D-dimer in 26 trials (55%), LDH in 24 trials (51%), lymphocyte count in 14 trials (30%), and IL-6 in 8 trials (17%). The cutoff values were variable for the included biomarkers. The most commonly used medications were tocilizumab, in 15 trials (32%), and anakinra in 10 trials (24.4%). This systematic review found high variability in CRS definitions and associated biomarker cutoff values in COVID-19 clinical trials. We call for a standardized definition of CRS, especially in COVID-19 patients.