Background Crohn's disease (CD) is frequently associated with malnutrition, inflammation and a deficiency of vitamin D (VD) with the relationships between these symptoms being poorly defined. VD is a modulator of the immune system and is associated with the onset of CD and disease activity. The level of serum VD may have potential in the assessment of CD activity. This study aimed to evaluate the relationships between VD, nutritional status and inflammation, and to identify more accurate VD thresholds. Methods The study included 76 outpatients with CD diagnosed between October 2018 and October 2020 and 76 healthy volunteers. Levels of serum 25(OH)D and nutritional indicators, as well as biochemical and disease activity assessments, were conducted. Results Patients with CD and healthy participants were found to differ significantly in their 25(OH)D levels as well in levels of nutritional and inflammatory indicators. The optimal VD cut‐off value was found to be 46.81 nmol/L for CD development and 35.32 nmol/L for disease activity. Levels of 25(OH)D were correlated with both nutritional status and inflammation. Conclusions The VD level is likely to be a useful additional tool in the evaluation of CD patients and predicting the disease activity and clinical response. The VD level may relate both to the nutritional status and levels of inflammation in CD patients, and disease progression. Highlights 1. This study included the NRS‐2002 and PG‐SGA scores to provide a comprehensive picture of the nutritional status with Crohn's disease (CD). 2. We found serum 25(OH)D levels were associated with both nutritional status and inflammation. 3. Receiver operating characteristic analysis was used to determine the optimal vitamin D cut‐off values for predicting CD development and assessing disease activity.