© 2019 by the authors. Licensee MDPI, Basel, Switzerland. Context: Obesity and low vitamin D status are linked. It is not clear that weight loss through lifestyle intervention is influenced by vitamin D status. Objective: The aim of this study was to investigate the effect of baseline vitamin D status and vitamin D supplementation on weight loss and associated parameters for participants on a weight loss program in a primary care setting. Design: A retrospective analysis of clinical records of patients who underwent an individually tailored weight loss program at a single dietetic clinic in Sydney, Australia. Setting: Primary care centers. Patients: 205 overweight and obese men and women aged from 18 to 50 years. Interventions: Patients were referred to a dietetic clinic for a weight loss program. Patients with low serum 25-hydroxyvitamin D (25(OH)D) concentrations at baseline were advised to increase sun exposure and take multivitamins supplemented with 2000 IU or 4000 IU per day of vitamin D3, according to the preference of their primary care physician. Main outcome measures: Clinical parameters of weight, height, waist circumference, and serum 25(OH)D, as well as blood pressure and fasting lipid profile were collected from both baseline and three-month follow-up consultations. Results: Subjects with sufficient baseline 25(OH)D levels (≥50 nmol/L) experienced significantly greater weight loss (−7.7 ± 5.9 kg vs. −4.2 ± 3.3 kg) and reductions in BMI (−2.6 ± 1.8 kg/m2 vs. −1.5 ± 1.1 kg/m2) and waist circumference (−5.2 ± 3.5 cm vs. −3.1 ± 3.1 cm) as compared with those who were vitamin D insufficient at baseline (p < 0.001 for all). Vitamin D insufficient patients who were supplemented with daily 2000 IU or 4000 IU vitamin D experienced significantly greater decreases in weight (−5.3 ± 3.6 kg vs. −2.3 ± 1.6 kg), BMI (−1.9 ± 1.2 kg/m2 vs. −0.8 ± 0.6 kg/m2) and waist circumference (−4.2 ± 3.4 cm vs. −1.2 ± 1.3 cm) as compared with those not supplemented (p < 0.001 for all). We also observed a greater decrease in low-density lipoprotein (LDL) cholesterol (−0.4 ± 0.5 mmol/L vs. −0.2 ± 0.5 mmol/L) in subjects insufficient at baseline and supplemented as compared with those insufficient at baseline and not supplemented (p < 0.01). Conclusion: In a weight loss setting in a dietetic clinic, adequate vitamin D status at baseline, or achieved at three months through supplementation, was associated with significantly greater improvement of anthropometric measures. The study has implications for the management of vitamin D status in obese or overweight patients undergoing weight loss programs.