The distillery stillage is a major byproduct generated during ethanol production from plant raw materials (e.g., cereals) and molasses. It contains a high percentage of organic matter susceptible to biodegradation and nutrients necessary for plant growth, and therefore, can be used for fertilization purposes. This study evaluated the fertilizing value of rye stillage applied in a grain maize agroecosystem. The field study was carried out in 2017–2018 (two growing seasons) on Luvisol (loamy sand) in Poland. The experiment scheme included four treatments: W0—the control with no stillage treatment, W15—15 m3 of stillage per hectare, W30—30 m3 of stillage per hectare, and W45—45 m3 of stillage per hectare. It was found that application of rye stillage was significant for maize yields. However, it demands supplementary potassium fertilization and regulation of the soil reaction and/or stabilization of the pH of stillage before its application. Moreover, due to its contribution to the build-up of residual available phosphorus in the soil, rye stillage may pose an environmental risk.