Purpose: This study assessed the effect of the number of remaining vertical walls on fracture resistance and marginal and internal adaptation of endocrown restorations. Materials and Methods: This in vitro study was conducted on 36 extracted mandibular molars. After endodontic treatment, the teeth were randomly divided into three groups (n=12) with four, three, and two remaining walls. The teeth were scanned, endocrowns were fabricated and seated, and their marginal and internal adaptation was assessed by measuring the gap at the margin (M1), center of cervical region (C1), cervico-axial angle (C2), three points at the axial wall (A1, A2, and A3), axiopulpal angle (P1), and center of pulpal floor (P2) by the replica technique. Fracture resistance was also measured. Data were analyzed by one-way ANOVA, and Tukey’s test (alpha=0.05). Results: The number of remaining walls significantly affected the gap at C1, C2, and P2. The mean gap at C1 in the 4-wall (P=0.002) and 2-wall (P=0.047) groups was significantly higher than that in the 3-wall group. The mean gap at C2 in the 4-wall group was also significantly higher than that in the 3-wall (P=0.017) and 2-wall (P=0.029) groups. The mean gap at P2 in the 4-wall group was significantly higher than that in the 2-wall group (P=0.037). The difference in fracture resistance was not significant among the groups (P=0.196). Conclusion: The number of remaining walls had no significant effect on fracture resistance, but the higher number of remaining walls may increase the internal and marginal gap of endocrowns.