Objectives: The aim of this research was to investigate the effectiveness and safety of a niacinamide serum, evaluate how the skin responds to the serum and check whether the application of the serum leads to improvements in skin parameters such as TEWL, hydration and erythema. Materials and methods: This was a randomized controlled clinical trial conducted at the University of Split School of Medicine in May and June 2023. The study aimed to assess the effectiveness and safety of a niacinamide serum on 25 healthy participants. Informed consent was obtained, and specific exclusion criteria were applied, including skin conditions, UV exposure, and pregnancy, among others. The forehead of the participants served as the test area, and the niacinamide serum (5%) was applied once daily for five weeks. Skin parameters were assesed non-invasively using the MPA6 device (Courage + Khazaka GmBH, Cologne, Germany) and included the measurements of transepidermal water loss (TEWL), skin erythema, and hydration levels. Additionally, subjective evaluations from participants regarding their experiences with the serum and any potential adverse effects were collected. The study was conducted under controlled conditions, maintaining constant temperature and humidity to ensure result accuracy. The research lasted five weeks, with a total of five measurement sessions conducted. Results: The use of niacinamide serum during the specified period resulted in a statistically significant increase in skin hydration on the side of the forehead where the niacinamide serum was applied. The hydration level increased from a baseline measurement of 56.45 AU to 62.02 AU at the fourth-week measurement, with a P-value of 0.047. There was also a reduction in transepidermal water loss and erythema, although it was not statistically significant. The serum did not induce any adverse effects on the treated side. Conclusion: The research has demonstrated that niacinamide serum has a positive impact on skin parameters. To obtain more objective results, it is necessary to include a larger number of participants from various age groups.