A simple, fast, selective, and robust analytical method for the determination of N-nitrososarcosine (NSAR) has been developed based on Supported Liquid–Liquid (SL–L) extraction coupled with Liquid Chromatography Tandem Mass Spectrometry (LC-MS/MS). To our knowledge, this is the first LC-MS/MS method for the analysis of NSAR in tobacco. In the past, NSAR was determined mainly by gas chromatography coupled with a thermal energy analyzer (GC-TEA), using a variety of sample cleanup and derivatization steps. The new method described here, using an isotope labeled internal standard, adds greater confidence for the determination of NSAR in complex sample matrices such as tobacco. It also uses a simpler extraction and cleanup procedure without the need for derivatization compared to existing methods. The method has been validated using three different control matrices (Kentucky Reference Cigarette KY 3R4F, smokeless research tobacco control products 1S2 dry snuff and 2S3 moist snuff), and the method exhibits good linearity (R² ≥ 0.999) over a wide range of concentration (3–2000 ng mL⁻¹). The limit of detection (LOD) is 27.3 ng g⁻¹ and the limit of quantification (LOQ) is 91.0 ng g⁻¹. While no NSAR was detected in KY 3R4F, and only a low amount in 2S3, a relatively larger amount of NSAR (550.5 ± 43.7 ng g⁻¹) was found in the dry snuff reference tobacco 1S2. The precision of the method is good (with a relative standard deviation of 7.94% for 1S2 samples, n = 100), and the method is robust and can be easily applied to the determination of NSAR in cigarette tobacco and smokeless tobacco products in a commercial laboratory environment.