To date, several methods for the quantification of tamoxifen and its metabolites have been developed, most of which employ liquid chromatography tandem–mass spectrometry (LC–MS/MS). These methods are highly sensitive and reproducible, but are also time‐consuming and require expensive equipment; one of their main disadvantages is matrix ionization effects. A more viable option, particularly in developing countries, is high‐performance liquid chromatography coupled with UV or fluorescence detection. We developed and validated a method for simultaneous quantification of tamoxifen, endoxifen and 4‐hydroxytamoxifen based on high‐performance liquid chromatography with fluorescence detection in a reverse‐phase column. The method is rapid (16 min plus 5 min of column re‐equilibrium), accurate (80–100%) and precise (0.23–6.00%), and does not require any additional irradiation process. Sample pretreatment consists of protein precipitation with acetonitrile under alkaline conditions, employing only 200 μL plasma. The validated method's wide range allowed quantification of steady‐state levels in patients under standard tamoxifen treatment (20 mg/day). This assay is ready for application in clinical studies and routine quantification of tamoxifen, endoxifen and 4‐hydroxytamoxifen in healthcare institutions.