Objective <p>This study systematically reviewed the impact of post diameter on treatment outcomes in endodontically treated teeth.</p> Materials and methods <p>The review followed the PRISMA guidelines, and a systematic search was conducted in the databases from 1975 to 2024. Publications were searched in PubMed, Wiley Online Library, EBSCOhost, Cochrane Library, and Scopus. A total of five studies were selected for inclusion after the screening process; three of which were cohort studies, one was prospective, and two case-control studies were included in the results. Data and outcome variables were extracted and checked for risk of bias using the ROBINS-I tool.</p> Findings <p>Studies that reported a decreased risk for failure and an increased survival rate for restorations with posts of greater diameter had a moderate risk of bias. Two studies reported a greater risk for complications resulting from an increase in post diameter and a reduction in remaining dentin thickness, but had a serious risk of bias. One study, in particular, reported an improvement in the survival rate of abutments upon increasing the post diameter; however, after a certain point, any increase led to the survival rate dropping by almost half.</p> Conclusions <p>Current evidence—largely from non-randomized studies—shows no clear association between post diameter and treatment survival, and remains limited by heterogeneity, risk of bias, and inadequate control of confounding factors.</p>