Dietary fiber (DF) is important as it benefits intestinal function, obesity, diabetes, dyslipidemia, and cancer. However, the extraction methods can modify the structural characteristics of DF, which could influence its functionality. On the other hand, there are no studies regarding the DF of pacaya inflorescences, which is a traditional food consumed by indigenous communities in Mexico after applying a thermal treatment. Therefore, the objective was to characterize through ATR-FTIR spectroscopy the structural changes in the FD of pacaya using three extraction methods. The thermal treatment was carried out using a sequential method (step 1 at 60 °C and 60 min; step 2 at 100 °C and 30 min). In the acid and alkali treatments, 5% w/v NaOH solution and 1% w/v citric acid solution were employed, respectively, at 40 °C for 120 min. Subsequently, the dispersion was filtered to obtain soluble dietary fiber (SDF) and insoluble dietary fiber (IDF) and then characterized using ATR-FTIR. The results showed changes in the intensity of the absorption bands of some functional groups related to the structure of cellulose, some hemicelluloses, pectins, and lignin. In particular, the alkaline treatment caused the hydrolysis of some of these components of the FD, such as cellulose, hemicelluloses, and pectins. Nevertheless, the modified DF of the tepejilote has great potential for application in the food industry.