The purpose of this study was to quantify polymorphs of active pharmaceutical ingredients in pharmaceutical tablets using a novel transmission low-frequency Raman spectroscopy method. We developed a novel transmission geometry for low-frequency Raman spectroscopy and compared quantitative ability in transmission mode versus backscattering mode using chemometrics. We prepared two series of tablets, (1) containing different weight-based contents of carbamazepine form III and (2) including different ratios of carbamazepine polymorphs (forms I/III). From the relationship between the contents of carbamazepine form III and partial least-squares (PLS) predictions in the tablets, correlation coefficients in transmission mode ( R2 = 0.98) were found to be higher than in backscattering mode ( R2 = 0.97). The root-mean-square error of cross-validation (RMSECV) of the transmission mode was 3.9 compared to 4.9 for the backscattering mode. The tablets containing a mixture of carbamazepine (I/III) polymorphs were measured by transmission low-frequency Raman spectroscopy, and it was found that the spectral shape changed according to the ratio of polymorphs: the relationship between the actual content and the prediction showed high correlation. These findings indicate that transmission low-frequency Raman spectroscopy possesses the potential to complement existing analytical methods for the quantification of polymorphs.