<p>The present study aimed to compare the performance of a 3D-endodontic guide (EG) and a conventional technique on the access of root canals containing fiberglass posts. Thirty single-canal lower human premolars were endodontically treated, filled using the single-cone technique, and prepared for fiberglass post luting. The teeth were positioned in the alveoli of human mandibles and cone beam computed tomography (CBCT) examination was performed. The specimens were randomly distributed (<i>n</i> = 10) according to the root canal access technique: control group (G_C)—no access; conventional access group (G_CA)—root canal accessed with spherical diamond burs and ultrasonic inserts; and EG group (G_EG)—root canal accessed with bone graft/anchorage drills and EG. At the end of the root canal access, new CBCT examination was performed to assess the root canal walls’ thickness and the volume of dental tissue removed during the fiberglass post removal. The time required for the access was also recorded. The data were submitted to statistical analysis (Kruskal–Wallis and <Emphasis Type="Underline">Mann–</Emphasis>Whitney tests,<i> α</i> = 0.05). G_CA promoted greater deviation than the G_EG (<i>p</i> &lt; 0.05) and resulted in greater removal of the root canal walls. The volume of dental tissue/fiberglass post removed, deviations promoted, and the time spent were significantly lower (<i>p</i> &lt; 0.05 and <i>p</i> &lt; 0.001, respectively) in G_EG than in G_CA. The use of the EG provided a lower volume of dental tissue removed, less deviation from the original root canal path, and less time for endodontic access when compared to conventional access technique.</p>