Simple SummaryQuality and safety are the most important features of poultry meat products for consumers. After the end of laying the meat of hens is hard, therefore methods of improving its tenderness are sought. Marinating is a frequently-used method used to improve the culinary qualities of a product. Synthetic additives for marinating meat are gaining increasingly less consumer acceptance. Acid whey and buttermilk are by-products of the dairy industry which are a source of many valuable components. The aim of this study was to evaluate marinating organic hen meat after the end of the laying period with buttermilk and whey for 24 and 48 h. In this article we have demonstrated that whey and buttermilk are suitable as natural marinades for marinating organic hen meat after laying period as they provide microbiological safety of the product and have a beneficial effect on tenderness and chewiness. In the sensory evaluation, they improve flavour and tenderness, which allows obtainment of a high-quality product. The material for the study was the breast muscles of hens after the laying period which were marinated with buttermilk and acid whey for 24 and 48 h. The quality parameters of non-marinated and marinated raw and roast products were evaluated in respect of physical traits (marinade absorption, pH, colour L*a*b*, shear force, TPA texture profile analysis test), microbiological parameters and sensory characteristics. The microbiological parameters were determined as the total viable counts of mesophilic aerobic bacteria of the Enterobacteriaceae family and Pseudomonas spp. Bacterial identification was performed by MALDI-TOF MS. The study showed that marinating the breast muscles of hens after the laying period with buttermilk and whey lightened the colour (p < 0.05), decreased the shear force value (p < 0.05), and reduced hardness and chewiness (p < 0.05) both after 24 and 48 h of marinating compared to the control product. The 24-h time of marinating with buttermilk and whey inhibited (p < 0.05) the growth of aerobic bacteria and Pseudomonas spp. and had a positive effect on the desirable odour, the intensity and desirability of flavour as well as the roast product tenderness. Longer marinating time reduced the product palatability and decreased its microbiological safety. The obtained results suggest that the 24-h time of marinating hen meat after the laying period with buttermilk and acid whey allows to obtainment of a high-quality product.