Jerusalem artichoke, a widely consumed edible, is an excellent source of inulin and selected phytochemicals. However, the improvement of its chemical composition by potassium fertilization has not yet been studied. Thus, the aim of the study was to evaluate the effect of different potassium (K) fertilization levels (K2O 150 kg ha&minus / 1, 250 kg ha&minus / 1, 350 kg ha&minus / 1) on the content of inulin / profile and changes in polyphenolic compounds / and the antioxidant capacity, including on-line ABTS antioxidant profiles of freeze-dried tubers originated from Violette de Rennes, Topstar, and Waldspindel cultivars. Inulin content was highest in the early maturing cv. Topstar. The application of 350 kg ha&minus / 1 of K fertilizer rates during the growth of cv. Topstar increased the inulin content of tubers by 13.2% relative to the lowest K fertilizer rate of 150 kg ha&minus / 1. In cv. Violette de Rennes, inulin accumulation increased in response to the fertilizer rate of 250 kg ha&minus / 1. A further increase in K fertilizer rates had no effect on inulin content. The inulin content of cv. Waldspindel was not modified by any of the tested K fertilizer rates. Thus, the accumulation of the inulin was cultivar-dependent. In the cultivars analyzed, 11 polyphenolic compounds were identified and polyphenolic compound content was affected by the applied rate of potassium fertilizer, which was dependent on the cultivar. Chlorogenic acid was the predominant phenolic acid in all cultivars, and it accounted for around 66.4% of the identified polyphenolic compounds in cv. Violette de Rennes and for around 77% of polyphenolic compounds in cv. Waldspindel and Topstar.