Chicken eggshells can be used as an attractive dietary source of mineral compounds, including calcium (Ca). However, the effects of chicken eggshell powder (CESP) on berry fruit juices have not been studied to date. Therefore, the objective of this study was to evaluate the effect of its addition to juices from chokeberry and cranberry on their phytochemical properties. The juices were determined for contents of polyphenols (determined by ultra-efficient liquid chromatography coupled with a mass detector (UPLC-PDA-ESI-MS/MS)), macro- and microelements (by inductively coupled plasma - optical emission spectrometry (ICP-OES)), and organic acids (by high-performance liquid chromatography (HPLC-PDA)) as well as for their antioxidative activity by radical scavenging capacity (ABTS) and ferric reducing antioxidative power (FRAP) assay, color profile (CIE L* a* b* system), and sensory attributes. The study results demonstrate that CESP addition to chokeberry and cranberry juices enriched them with minerals and increased their Ca content 25.7 times and 66.3 times, respectively, compared to the control samples. Juices supplementation with CESP significantly decreased their acidity and total organic acids content as well as increased their pH value. Chokeberry and cranberry juices supplementation with 1% CESP caused no significant changes in the amount of precipitate and their color, but it significantly improved their taste. For this reason, CESP addition in the amount of up to 1% can be suggested as the optimal supplementation of berry fruit juices. The study also demonstrated that CESP addition in the amount of up to 1% caused no significant differences in the content of polyphenolic compounds and in the antioxidative activity of juices, which can be deemed important from the viewpoint of their putative health benefits. In addition, the heat treatment of juices contributed to only a 4% loss of polyphenolic compounds from the CESP-supplemented juices compared to the 6% loss from the non-supplemented juices.