Purpose <p>Dental implants are a reliable treatment for edentulism, but peri-implant diseases remain a challenge. Vitamin D’s role in bone remodeling and immune function suggests it may influence peri-implant health. This study aims to compare serum vitamin D levels between patients with peri-implant mucosal inflammation and healthy peri-implant tissues.</p> Materials and methods <p>In this cross-sectional study, 47 patients with functional dental implants were classified into peri-implant health, mucositis, and peri-implantitis groups using established clinical criteria. Clinical and radiographic assessments, including probing depth, plaque index, bleeding on probing, and bone loss, were performed. Serum vitamin D levels were measured from blood samples (using the High-Performance Liquid Chromatography (HPLC) method with assay kit (Technocrom, Novin Tajhiz Gostar Laleh Co., Tehran, Iran)). Data analysis was conducted with SPSS, applying appropriate statistical tests and considering <i>p</i> &lt; 0.05 as significant.</p> Results <p>Of the 47 participants, serum vitamin D levels were significantly lower in the peri-implantitis group compared to peri-implant health (<i>p</i> = 0.003). Vitamin D insufficiency was observed exclusively in peri-implant mucositis and peri-implantitis cases, with the highest prevalence in peri-implantitis. Lower vitamin D levels were significantly associated with increased bleeding on probing and greater bone resorption, particularly at 3&#xa0;mm, 4&#xa0;mm, and 5&#xa0;mm (<i>p</i> &lt; 0.05). No significant associations were found between vitamin D levels and probing depth or local plaque index (<i>p</i> &gt; 0.05).</p> Conclusion <p>Lower serum vitamin D levels were significantly associated with peri-implantitis and greater peri-implant bone loss in this study.</p>