Objectives <p>This study aims to measure and compare the effects of immediate pre-endodontic dentin sealing (IPDS) and late dentin hybridization (LDh), combined with two different irrigation protocols (NaOCl/EDTA or normal saline), on cuspal deflection (CD) and fracture strength (FS) of endodontically treated teeth. </p> Materials and methods <p>Sixty maxillary first premolars were divided into four experimental groups (A1: LDh + NS, A2: LDh + NaOCl/EDTA, B1: IPDS + NS, B2: IPDS + NaOCl/EDTA) and two control groups (C: sound teeth, D: unrestored teeth). Teeth underwent standardized MOD cavity preparation, endodontic treatment, and restoration, except for group D was left without restoration. CD was measured after cavity preparation, 15&#xa0;min post-restoration, and after thermocycling. FS was assessed using a universal testing machine. </p> Results <p>No significant CD differences were found after cavity preparation. However, 15&#xa0;min post-restoration, LDh groups (A1, A2) exhibited significantly higher CD than IPDS groups (B1, B2), with A2 group CD being higher than A1 group. After thermocycling, IPDS groups demonstrated greater cuspal relaxation compared to LDh groups. FS was highest in sound teeth (1533.2N) and lowest in unrestored teeth (571.5N). IPDS groups had significantly higher FS than LDh groups, with no significant difference between B1 and B2 groups. </p> Conclusions <p>The study concluded that IPDS reduces CD and enhances FS compared to LDh, suggesting it as an effective step before endodontic treatment. NaOCl/EDTA irrigation influenced CD and FS in LDh groups but had no significant impact in IPDS groups.</p> Clinical relevance <p>Dentin sealing before root canal treatment and prior to exposure to chemical substances (such as irrigation with NaOCl) enhances the structural integrity of endodontically treated teeth. This protective measure protects the freshly cut dentin from the detrimental effects of these substances, thereby improving the bonding strength and overall clinical outcomes.</p>