<p>Tamoxifen is a nonsteroidal selective estrogen receptor modulator (SERM), meaning that it acts as an anti-estrogen at estrogen receptors in certain tissues (breast tissue), while acting as an estrogen in other tissues (bone tissue and endometrium). The aim of this work was to establish thin-layer chromatography (TLC) conditions for the separation of <i>Z</i>-isomer of tamoxifen from its impurity F (<i>N</i>-demethyltamoxifen) and to validate the TLC method for the assay determination of impurity F in dosage forms. The method was validated according to the International Council for Harmonisation (ICH) Q2(R2) guidelines. Samples were spotted onto the TLC silica gel G F₂₅₄ plate and developed in the saturated twin-trough chamber. The selected mobile phase was toluene‒cyclohexane‒triethylamine (65:25:20, <i>V/V</i>). The migration distances (28&#xa0;mm and 64&#xa0;mm for <i>N</i>-demethyltamoxifen and tamoxifen, respectively) with low relative standard deviation (RSD) values (2.59%) showed satisfactory reproducibility of the chromatographic system. The TLC Scanner was used for direct evaluation of the chromatograms in reflectance/absorbance mode. The calibration curves were generated (<i>r</i> = 0.998). The precision and detection limits as well as the recovery values (99.16‒101.02%) were validated and found to be satisfactory. On the basis of these results, it can be concluded that the developed TLC method is a rapid and efficient method for testing the purity of tamoxifen in bulk and pharmaceutical dosage forms.</p>