In-syringe ionic liquid dispersive liquid–liquid microextraction was developed and applied for the extraction of sulfonamides in blood. High-performance liquid chromatography was applied for the determination of the analytes. The parameters affecting the extraction efficiency, including volume of ionic liquid, salt concentration, pH value of the system, extraction time, extraction temperature and cooling time, were investigated. The limits of detection for sulfadiazine, sulfamethyldiazine, sulfamethizole, sulfachlorpyridazine, sulfamethoxazole and sulfisoxazole were 12.30, 2.22, 3.19, 1.67, 2.93 and 1.58 ng mL⁻¹, respectively. When the present method was applied to the analysis of blood samples, the recoveries of the analytes ranged from 87.0 to 115.3% and the relative standard deviations were lower than 7.2%.