Objectives: This study aimed to compare the fit accuracy between retainers fabricated using conventional cold-curing resin (hereinafter referred to as “conventional retainers”) and those fabricated using three-dimensional (3D) printing based on computer-aided design/computer-aided manufacturing (CAD/CAM) technology (hereinafter referred to as “CAD/CAM retainers”). Furthermore, the study aimed to compare two different methods to evaluate the fit accuracy: the impression replica technique and the 3D triple-scan protocol. Methods: For each of the 20 working models derived from a maxillary typodont, one conventional retainer and one CAD/CAM retainer were fabricated. The fit accuracy was evaluated using the impression replica technique and the 3D triple-scan protocol. Measurements were taken at 12 points on each model, and the differences in thickness (gap) were analyzed using Wilcoxon’s signed-rank test. Moreover, the correlation between thickness and measurement site was evaluated using Spearman’s rank correlation coefficient. Results: In both evaluation methods, the CAD/CAM retainers exhibited superior fit accuracy compared to the conventional retainers. Notably, the 3D triple-scan protocol clearly demonstrated that the fit accuracy differed depending on the measurement site. Conclusions: CAD/CAM retainers demonstrated superior fit accuracy compared to conventional retainers, possibly because digital design can account for polymerization shrinkage. In the impression replica technique, the median (interquartile range) thickness for the conventional retainers was 0.169 (0.120–0.260) mm, whereas that for the CAD/CAM retainers was 0.136 (0.096–0.198) mm. The CAD/CAM retainers showed significantly smaller gap values (p < 0.001). Within the limitations of this in vitro study, CAD/CAM retainers showed significantly smaller gap values than conventional retainers, indicating improved fit accuracy. In particular, the 3D triple-scan protocol accurately captured site-specific variations in fit accuracy among the anterior, canine, and molar regions.