Background <p>Bulk-fill flowable resin composites have demonstrated clinical effectiveness in restoring Class I and II cavities in vital teeth. However, their performance in endodontically treated teeth remains unclear. This randomized, controlled, and double-blind pilot clinical trial aimed to evaluate the one-year clinical performance of direct restorations in endodontically treated premolars, using either a conventional incremental technique or a technique incorporating a bulk-fill flowable composite base.</p> Methods <p>Nineteen patients with at least one endodontically treated maxillary premolar requiring a direct restoration were enrolled. A total of 21 premolars were restored with either a nanohybrid resin composite (VT group) or a bulk-fill flowable composite base followed by nanohybrid composite (VT-F group). Two calibrated examiners evaluated restorations at baseline, 6 months, and 12 months using modified USPHS criteria. Mann–Whitney U and Wilcoxon signed-rank tests were applied for intergroup and intragroup comparisons (α = 0.05).</p> Results <p>Compared with baseline, the VT group showed a significant increase in surface roughness at 6 and 12 months (<i>p</i> = 0.025). No significant differences were observed between groups in other evaluated parameters, including anatomic form, marginal discoloration, retention, marginal adaptation, color match, and secondary caries (<i>p</i> &gt; 0.05). One restoration in the VT-F group failed due to fracture at the 6-month recall.</p> Conclusions <p>After one year, the clinical performance of endodontically treated premolars restored with a bulk-fill flowable base was comparable to that of restorations placed using the conventional incremental technique.</p> Trial registration <p>Brazilian Registry of Clinical Trials (RBR-107w28wf). Registered on 10/10/2022. Retrospectively registered.</p>