Boswellia serrata and Boswellia sacra (syn. B. carteri) are important medicinal plants widely used for their content of bioactive lipophilic triterpenes. The qualitative and quantitative determination of boswellic acids (BAs) is important for their use in dietary supplements aimed to provide a support for osteoarthritic and inflammatory diseases. We used High Performance Liquid Chromatography (HPLC)-Diode Array Detector (DAD) coupled to ElectroSpray Ionization and tandem Mass Spectrometry (ESI-MS/MS) for the qualitative and quantitative determination of BAs extracted from the gum resins of B. sacra and B. serrata. Limit of detection (LOD), limit of quantification (LOQ), and Matrix Effect were assessed in order to validate quantitative data. Here we show that the BAs quantitative determination was 491.20 g·kg−1 d. wt (49%) in B. sacra and 295.25 g·kg−1 d. wt (30%) in B. serrata. Lower percentages of BAs content were obtained when BAs were expressed on the gum resin weight (29% and 16% for B. sacra and B. serrata, respectively). The content of Acetyl-11-Keto-β-Boswellic Acid (AKBA) was higher in B. sacra (70.81 g·kg−1 d. wt; 7%) than in B. serrata (7.35 g·kg−1 d. wt; 0.7%). Our results show that any claim of BAs content in either B. sacra or B. serrata gum resins equal to or higher than 70% or AKBA contents of 30% are simply unrealistic or based on a wrong quantitative determination.