The purpose of this study was to evaluate the effects of vitamin D supplementation on ovarian reserve markers, including serum anti-Mullerian hormone (AMH) level, follicle-stimulating hormone (FSH) level, and antral follicle count (AFC), in infertile women with diminished ovarian reserve and vitamin D deficiency.A prospective, nonrandomized, cross-sectional study was conducted. Women aged 18 to 41 years who were unable to become pregnant after 12 months of sexual intercourse and had normal tubal patency, partners with normal semen analysis, diminished ovarian reserve, and 25-hydroxyvitamin D [25(OH)D] deficiency were included. Eligible patients' AFC and serum levels of AMH, FSH, 25(OH)D, 1,25-dihydroxyvitamin D [1,25(OH)D], calcium, phosphate, alkaline phosphatase, and parathormone were assessed before and after administration of 300,000 IU of vitamin D ampules. Changes in the parameter values after vitamin D supplementation were compared with the initial levels.The study was conducted in 62 of the 142 participants. The AFC and AMH, 25(OH)D, 1,25(OH)D, phosphate (P < .01), and calcium levels (P < .05) were statistically significantly increased after vitamin D supplementation. Statistically significant decreases in FSH (P < .01) and alkaline phosphatase levels (P < .05) were observed after vitamin D supplementation. No statistically significant correlations were found between 25(OH)D level and AFC, 1,25(OH)D level, AMH level, and FSH level before and after supplementation (P > .05).As improvements in the ovarian reserve markers were obtained with vitamin D supplementation, vitamin D might be considered as a fertility treatment for patients with diminished ovarian reserve and vitamin D deficiency. Abbreviations: [1,25(OH)D] = 1,25-dihydroxyvitamin D, [25(OH)D] = 25-hydroxyvitamin D, AFC = antral follicle count, ALP = alkaline phosphatase, AMH = anti-Mullerian hormone, BMI = body mass index, COS = controlled ovarian stimulation, DOR = diminished ovarian reserve, FSH = follicle-stimulating hormone, IVF = in vitro fertilization, PCOS = polycystic ovary syndrome.