This study aims to evaluate the effect of different endodontic sealers (epoxy resin, eugenol, and bioceramic/calcium silicate-based) and the time of cementation (immediately or 7 days after canal obturation) on the bond strength of a fiberglass post cemented with RelyX™ ARC. Eighty-four premolars were instrumented and divided into groups (n = 12) according to the sealer and the time of post cementation: Endofill (EN), Endosequence BC Sealer (BC), and AH Plus (AH) had immediately fiber post cementation; EN7, BC7, and AH7 had post cementation after 7 days; and control group (C) had fiber post cementation without endodontic sealer. Each post space of the root was cut into slices and submitted to push-out test. Failure mode was assessed. Two-way ANOVA, Tukey’s, and Dunnett’s tests were used for statistical analysis (α = 5%). The type of endodontic sealer (p < 0.001), the time of post cementation (p = 0.038), and the interaction sealer time (p = 0.002) had negative influence on bond strength of fiberglass posts cemented with RelyX™ ARC. AH promoted the highest bond strength mean values (21.20 MPa immediately and 15.54 MPa at 7 days). EN (9.75 MPa immediately and 13.15 MPa at 7 days) and BC (10.43 MPa immediately and 5.73 MPa at 7 days) had lower bond strength than AH, regardless the time of cementation. AH was the best sealer to obturate the root canal when fiberglass cementation with resin-based cement is planned. The correct choice of an endodontic sealer and the adequate time of post cementation may avoid post dislocation caused by low bond strength to dentin.