Cognitive enhancement through stimulants such as modafinil is becoming increasingly popular, with many individuals using prescription stimulants for non-medical purposes to improve alertness, attention, and mood. The misuse of such substances has raised concerns, particularly in forensic toxicology. The UHPLC-QqQ-MS/MS method was developed to quantify modafinil in evidentiary samples and biological materials. Additionally, the authors noted the presence of sodium adducts during the analysis of samples with high concentrations of modafinil. The method was validated for accuracy, precision, and linearity, with a concentration range of 0.1-10.0 µg/mL for the evidences and 1.0-100.0 ng/mL for blood. The method successfully detected modafinil as the sole substance in all evidences, with concentrations ranging from 90.7 to 120.8 mg, corresponding to 45.5% to 80.5% of the labeled dose. The method was applied to real post-mortem human cases, where, among others, the concentration of modafinil in blood was 110 ng/mL, whereas, in another case, the concentration of modafinil in the putrefaction fluid exceeded 1000 ng/mL. The developed UHPLC-QqQ-MS/MS method is effective for the quantification of modafinil in evidentiary samples and biological materials, offering a reliable tool for forensic toxicology applications. This method can be used to evaluate modafinil use in both legal and illicit contexts, including cases of overdose or misuse.