A sensitive and rapid LC–MS/MS method was developed and validated for the quantification of SUVN-502 and M1 of SUVN-502, a 5-HT6 receptor antagonist for the treatment of dementia associated with Alzheimer’s disease. Following solid-phase extraction, SUVN-502 and M1 of SUVN-502 and IS were eluted with 10 mM ammonium acetate (pH 4.0) and acetonitrile using a rapid gradient program on reverse phase column. Multiple reaction monitoring mode was used to monitor the respective transitions of m/z 478.2 → 377.7 for SUVN-502 and m/z 464.1 → 377.7 for M1 of SUVN-502. The assay exhibited a linear dynamic range of 10–10000 pg/mL for SUVN-502 and 20–20000 pg/mL for M1 of SUVN-502 in human plasma. Acceptable precision and accuracy were obtained for concentrations over the standard curve range. The within batch accuracy and precision were within acceptable limits. All the other validation parameters were within the acceptable limits. The validated method was applied to analyze human plasma samples obtained from a human pharmacokinetic study consisting single and multiple ascending doses.