: The pathogenesis of intensive care unit-acquired weakness (ICU-AW) is multi-factorial, with some of the main risk factors being sepsis, multiorgan failure, and the inflammatory response related to critical illness. Vitamin D is crucial for muscle function, the immune response, and inflammation, and has been identified as a predictor of negative outcomes in intensive care unit (ICU) patients with COVID-19. The objective of this preliminary study was to examine the relationship between vitamin D serum levels and the incidence of ICU-AW in a cohort from the University Hospital of Split. : A prospective observational cohort study was conducted in the University Hospital of Split in ICU from December 2021 to March 2022. The inclusion criteria were as follows: patients over 18 years old who had a confirmed severe acute respiratory coronavirus disease 2 (SARS-CoV-2) infection, patients who were mechanically ventilated for more than 48 h, and patients who were weaned from a ventilator over at least 24 h. The exclusion criteria were a history of neurological or musculoskeletal disorders and a pre-existing poor functional status. Vitamin D was detected in the first routine blood sample. : A total of 77 patients were observed, with 36 patients who were successfully weaned from a ventilator over at least 24 h and 1 patient who could not be examined because of impaired consciousness (this patient was excluded from further analysis), and thus a total of 35 patients were analyzed. Of these 35 patients, 12 (34%) developed ICU-AW. The median vitamin D serum level in the ICU-AW group was 17 (7.5-73.3), while that in the non-ICU-AW group was 25.2 (12.3-121). The difference in vitamin D serum levels between the groups was not significantly different from zero ( = 0.567). All patients, except for one, were vitamin D insufficient. : Vitamin D serum levels in the ICU-AW group were not statistically different from the non-ICU-AW group, possibly due to the small sample size. Given the known roles of vitamin D in muscle function, immune modulation, and inflammation, a potential etiopathogenetic role in ICU-AW cannot be excluded without additional studies. Therefore, further studies with larger sample sizes than ours are necessary to determine whether vitamin D deficiency contributes to the development of ICU-AW and whether supplementation could have preventive or therapeutic value.