Capivasertib is an oral molecular targeted drug for treating hormone receptor-positive, human epidermal growth factor receptor 2-negative inoperable or recurrent breast cancer with phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha, AKT serine/threonine kinase 1, and phosphatase and tensin homolog mutations. Moderate intra- and inter-individual variabilities have been reported in the pharmacokinetic study of capivasertib. Furthermore, because capivasertib is a substrate and weak inhibitor of CYP3A4, drug–drug interactions are expected in elderly patients with breast cancer due to the concomitant use of multiple drugs for co-morbidities. These factors suggest that the therapeutic drug monitoring (TDM) of patients receiving capivasertib may improve therapeutic efficacy while decreasing the onset of adverse events. However, to the best of our knowledge, there are no reports on methods to measure the capivasertib concentration in human plasma. Therefore, this study proposes a method for determining the capivasertib concentration in human plasma using HPLC-UV. Following protein precipitation with methanol, capivasertib and pirfenidone (internal standard) were separated using isocratic elution on a C18 column using a mobile phase of 0.5% KH₂PO₄ (pH 4.5)/acetonitrile in a 73:27 (vol/vol) ratio, pumped at a constant flow rate of 1.0 mL/min. Quantification was performed at 219 nm. The calibration curves were linear over the range of 50–1000 ng/mL. Intra- and inter-day coefficients of variation were less than 10.2%. The assay accuracy ranged from -7.2–2.9%, and the recovery was >93.8%. This simple and cost-effective method may contribute to the TDM of capivasertib in clinical practice.