Olaparib is a potent PARP inhibitor in clinical use for cancer therapy. A bioanalytical assay was developed and validated for quantitation of intracellular level of olaparib in cells exposed to the drug. The assay involves an optimized and straightforward sample pretreatment with acetonitrile for olaparib solubilization, cell lysis and protein precipitation, and a high performance liquid chromatography (HPLC) method with ultraviolet detection. Several parameters in both the sample preparation and the detection steps were investigated. Optimal chromatographic conditions were achieved with a 5 μL injection on a Nova-Pak® C18 column (150 × 3.9 mm, 4 μm) using a mobile phase consisting of acetonitrile and ultra-pure water in gradient mode, at a constant 1.2 mL/min flow rate, at 35 °C. Detection was carried out at 254 nm and a diode array detector was used to insure purity of the olaparib peak. The method was validated according to Food and Drug Administration guidelines. Linearity, accuracy and precisions were satisfactory over the concentration range of 200–2000 ng/mL. Limits of detection and quantification for olaparib were 50 ng/mL and 200 ng/mL, respectively. Good stability was showed in three relevant analytical conditions. Finally, the validated analytical method was successfully used to estimate the intracellular level of olaparib in SUM1315 breast cancer cells. A significant difference was observed in intracellular drug level after 1 and 3 h incubations. This method permitting measurement of drug level in tumor cells would allow dosage optimization and improvement of treatment response predictions.