Oxidative changes in fish feed during storage are a major problem in aquaculture nutrition. The current study aims to present fast and simple spectrometric method to measure thiobarbituric acid reactive substances (TBARS) in the commercial rainbow trout feeds without hydrolysis step, as a method for determination of lipid peroxidation. The proposed test was validated and relevant method characteristics which were evaluated within the validation protocol were: linearity (range), limit of detection, limit of quantification, precision (repeatability and intra-laboratory reproducibility), accuracy and method performance (X- and R-control charts). The obtained results indicated that presented TBARS test was linear in the range of 1.11–13.33 nmol/mL and precise under both repeatability and reproducibility test conditions. The method was characterized by acceptable accuracy considering low analyte concentration in the samples. The method also showed to be stable in terms of changes in the average level and variability. The results in the present work indicated that the proposed test could be successfully applied for TBARS quantification in rainbow trout feed for routine quality control in aquaculture sector, especially in case when need to analyse a great number of samples in a short time.