A sensitive, specific, selective and rapid LC-ESI–MS/MS method has been developed and validated for the quantification of apalutamide in mice plasma using apalutamide-d3 as an internal standard (I.S.). Sample preparation was accomplished through a simple protein precipitation process. Chromatography of apalutamide and the I.S. was achieved on an Atlantis dC18 column using an isocratic mobile phase comprising 0.2% formic acid in water and acetonitrile (20:80, v/v) delivered at a flow rate of 0.8 mL/min. LC–MS/MS was operated under the multiple reaction-monitoring mode (MRM) using the electrospray ionization technique in positive ion mode and the transitions of m/z 478 → 450 and m/z 481 → 453 were used to measure the derivative of apalutamide and the I.S, respectively. The total chromatographic run time was 2.5 min and the elution of apalutamide and I.S. occurred at 1.10 and 1.09 min, respectively. Method validation was performed as per regulatory guidelines and the results met the acceptance criteria. Linearity was established in the concentration range of 1.02–2030 ng/mL (r> 0.995) for apalutamide. The intra- and inter-day accuracy and precision for apalutamide in mice plasma were in the range of 2.11–8.44 and 2.51–6.09%, respectively. Apalutamide was found to be stable under various stability conditions. This novel method has been applied to a pharmacokinetic study in mice.