Apical periodontitis (AP) is a common dental condition that can be influenced by diabetes mellitus. This study aimed to investigate the impact of type 2 diabetes mellitus (T2DM) on the prevalence and severity of AP, considering the adequacy of endodontic treatments. A total of 180 patients selected based on specific dental criteria from a private clinic in Oradea, Romania, were included in this study. Clinical data were collected through medical records and panoramic radiographs. Statistical analyses were conducted using SPSS software, employing the Fisher test, Mann-Whitney test, and binary logistic regression to determine correlations between T2DM and AP in both endodontically treated and non-treated teeth. Additionally, we examined the relationship between diabetes and AP in teeth that received adequate root canal treatment (RCT), as well as the correlation of AP with the adequacy of endodontic treatment. Non-treated teeth were significantly more likely to have AP in patients with T2DM than in non-diabetic patients (OR = 5.3, P < 0.001). No significant difference in AP prevalence was observed between treated teeth in diabetic and non-diabetic patients. Inadequate RCT was associated with a higher incidence of AP, regardless of diabetes status (OR = 26.9, P < 0.001). The study concludes that DM significantly increases the risk of AP in untreated teeth, with diabetic patients showing a higher prevalence of AP than non-diabetic patients. However, this increased risk is not observed in adequately treated teeth. The quality of RCT is critical, as inadequate RCT is linked to a higher incidence of AP, regardless of diabetes status.