Background <p>Apical voids reduce cement thickness uniformity and increase the risk of post dislodgment. This study evaluated the pull-out force, resin cement thickness, and the volume of voids and gaps in the apical third of two fiberglass posts (Exacto and Reforpost^®) luted with RelyX™ ARC (R-ARC) or RelyX™ U200 (R-U200).</p> Methods <p>Thirty-two endodontically treated single-rooted teeth were assigned to two post groups (Exacto, Reforpost^®; <i>n</i> = 16), each subdivided according to the resin cement used (R-ARC or R-U200; <i>n</i> = 8). Micro-CT was used to quantify resin cement thickness and apical voids/gaps. Pull-out testing and failure mode analysis were performed. Voids/gaps and pull-out force were analyzed using two-way ANOVA/Tukey, resin cement thickness with three-way ANOVA/Tukey, and failure mode with chi-square.</p> Results <p>Reforpost^® showed significantly higher pull-out force and thinner cement layers than Exacto (<i>p</i> &lt; 0.05). With R-ARC, the void volume did not differ between posts (<i>p</i> &gt; 0.05), whereas R-U200 used with Reforpost^® produced significantly fewer voids than with Exacto (<i>p</i> &lt; 0.05). R-ARC consistently exhibited voids at the post tip and gaps at the post–dentin interface, while R-U200 showed minimal or no voids and improved apical adaptation.</p> Conclusions <p>The parallel-sided serrated post (Reforpost^®) combined with a self-adhesive dual-cure resin cement (RelyX™ U200) resulted in superior apical adaptation, fewer voids, and higher pull-out strength compared with the smooth tapered post (Exacto) or the conventional cement (RelyX™ ARC). Clinically, selecting a post with better anatomical conformity, together with a self-adhesive resin cement, may enhance post retention and reduce the risk of debonding in endodontically treated teeth.</p>