A novel method has been established for the rapid separation and determination of free fatty acids from 37 different varieties of raspberry. In this study, a new fluorescent labeling reagent for fatty acids, 2-(4-amino)-phenyl-1-hydrogen-phenanthrene [9, 10-d] imidazole (PIA), has been synthesized and successfully applied to the high-performance liquid chromatography (HPLC) determination of fatty acids in raspberry. The novel method has been optimized by HPLC with fluorescence detection and online mass spectrometry identification (HPLC-FLD-MS/MS). The 22 main fatty acids (FAs) present in raspberry were derivatized by PIA and separated on a reversed-phase Hypersil GOLD column with gradient elution. The main experimental parameters affecting extraction efficiency and derivatization yield were investigated and optimized by response surface methodology (RSM) combined with Box-Behnken design (BBD). Under the optimum conditions, the method was successfully applied for the analysis of 22 fatty acids in 37 different varieties of raspberry. Good linear correlations were observed for all fatty acids with correlation coefficients of > 0.9978. Limits of detection and quantification (LOD and LOQ) were in the range of 0.12 to 0.49 ng/mL and 1.07 to 2.81 ng/mL, respectively. Furthermore, the results indicated that the raspberries were rich in fatty acids, but the contents of the fatty acids varied among the different varieties.