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Background: Coronal restoration of endodontically treated teeth (ETT) with mesio-occluso-distal (MOD) cavities is of a great importance in long-term success of the treatment. This study evaluated the effect of fiber reinforcement on the fracture resistance (FR) of ETT restored with flowable or paste bulk (PB)-fill composite resin compared to conventional composite (CC) resin. Materials and Methods: In this in vitro experimental study, eighty maxillary premolars were divided into eight groups (n = 10). The first group was left intact (G1) and the other groups received MOD cavities along with endodontic treatment. G2: Remained unrestored while the other experimental groups were restored with three types of composite resin with or without fiber insertion. G3: CC resin, G4: PB fill, G5: Flowable bulk fill (FB). G6: Fiber + CC, G7: Fiber + PB, and G8: Fiber + FB. FR was tested at crosshead speed of 1 mm/min and recorded in Newton. Data were analyzed using one-way analysis of variance and Tukey's tests at significance level of P < 0.05. Results: G1 and G2 revealed the highest and the lowest FR, respectively. The mean FR of the testing groups in Newton was as follows: G1 = 1204.8 A, G2 = 352.1 C, G3 = 579.6 BD, G4 = 596.7 BD, G5 = 624.9 BDE, G6 = 858.3 E, G7 = 529.6 CB, and G8 = 802.5DE. Different uppercase letters indicate the significant difference between the groups. Conclusion: The effect of fiber insertion on FR depended on the type of composite resin; the highest reinforcing effect was obtained in the CC resin + fiber, followed by bulk-fill flowable + fiber, and flowable bulk (FB)-fill composite resin. The strength of the former was significantly higher than the conventional and PB fill with and without fiber.