The process of oxidation within the human body causes damage to various cellular components. During this process, free radicals are created which cause cellular damage. Antioxidants are compounds that prevent damage within cells by neutralizing the free radicals or by interfering in chain reactions involving free radicals. Despite the reputation of Japanese knotweed (Fallopia japonica) as the most invasive species, it has been shown to be a high source of different bioactive compounds with potential antioxidant activity. Lactic acid fermentation can be used to improve the nutritional as well as functional value of substrates including antioxidant activity. This study aimed to compare the impact of non-fermented and fermented Japanese knotweed on oxidation level in yeast Saccharomyces cerevisiae. The fermentation of F. japonica was carried out by Lactobacillus plantarum and evaluated by measuring pH values and L. plantarum (LAB) growth. In ethanol extracts prepared from fermented and non-fermented F. japonica, total phenolic content using the Folin- Ciocalteu reagent, antioxidant activity by DPPH assay, and antioxidant activity in the cells were measured. The maximum decrease in pH value is observed in the first 24 hours. However, the maximum LAB growth is seen in the first 48 hours. Total phenolic content decreased in 48 hours of fermentation, but DPPH radical scavenging activity increased in the first 48 hours of fermentation. The antioxidant capability of F. japonica was further proved with the decrease of oxidation level in cells of yeast Saccharomyces cerevisiae. Therefore, the biomass of F. japonica has demonstrated its potential as a source of antioxidants.