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Aim: The aim of this study was to evaluate the exogenous oral administration of vitamin D3, at the serum 25(OH)D levels, force parameters and bone mineral density (BMD), in professional soccer players. Subjects/Methods: Forty two professional soccer players (Α´ National League) were assessed for the levels of serum vitamin D3, in order to evaluate the prevalence of vitamin D3 deficits in professional soccer players. For the participants that had significant deficits (vitamin D3 levels < 30nmol/l), an exogenous administration protocol was designed that involved the administration of 7000 IU vitamin D3/week for 9 months (August-April). Serum 25(OH)D levels, BMD, muscle function (isokinetic performance and vertical jump) and the ratio of free testosterone/cortisol, were measured in three periods: right before the vitamin D3 adjuvant intake, 5 and 9 months after. Results: The concentration of serum 25(OH)D was 27.64±4.54, 32.15±12.05, 36.54±10.6 nmol/l (mean ± SD), before, 5 and 9 months after intake, respectively. Serum 25(OH)D levels significantly increased throughout the whole 9 months p=0.021). At the same time, there was a significant increase (p=0.016) in BMD, with mean levels being significantly higher after the 9 months of the study, in comparison with baseline levels (1.465±0.077 g/cm2vs 1.45±0.076 g/cm2). Finally, there were no significant differences in any of the force tests, or in the ratio of free testosterone/cortisol in the 9month study duration (p>0.05). Conclusions: There are two main findings according to the findings. Firstly, the use of vitamin D3 supplement is associated with increased serum 25(OH)D levels and BMD. Secondly, the exogenous administration of vitamin D3 did not seem to improve force parameters nor free testosterone/cortisol ratio in professional soccer players.