Purpose <p>To our knowledge, no previous reports have described the postmortem distribution of thiamethoxam in fatal cases. This study aimed to quantify thiamethoxam in biological fluids and solid tissues from a suicide involving ingestion of Actara^® insecticide (10% thiamethoxam) using liquid chromatography–tandem mass spectrometry (LC–MS/MS).</p> Methods <p>Biological fluids and solid tissue homogenates were deproteinized with methanol. Thiamethoxam-d4 served as the internal standard. The standard addition method was used for quantification.</p> Results <p>Thiamethoxam was detected in all analyzed specimens, including 4 biological fluids, 11 solid tissues, and stomach contents. Thiamethoxam concentrations in each specimen, excluding the stomach contents, ranged from 2.5 to 53.6&#xa0;µg/mL(g). The correlation coefficients (<i>r</i>²) for the calibration curves ranged from 0.9934 to 0.9996. Intraday and interday variability were &lt; 8.9% and &lt; 11.1%, respectively.</p> Conclusions <p>Thiamethoxam was quantified in biological fluids and solid tissues obtained at autopsy using LC–MS/MS with standard addition. To our knowledge, this is the first detailed report on the postmortem distribution of thiamethoxam.</p>