Abstract Improving and stabilizing the quality of seed proteins are of growing interest in the current food and agroecological transitions. Sulfur is a key determinant of this quality since it is essential for the synthesis of sulfur-rich proteins in seeds. A lack of sulfur provokes drastic changes in seed protein composition, negatively impacting the nutritional and functional properties of proteins, and leading in some cases to diseases or health problems in humans. Sulfur also plays a crucial role in stress tolerance through the synthesis of antioxidant or protective molecules. In the context of climate change, questions arise regarding the trade-off between seed yield and seed quality with respect to sulfur availability and use by crops that represent important sources of proteins for human nutrition. Here, we review recent work obtained in legumes, cereals, as well as in Arabidopsis, that present major advances on: (i) the interaction between sulfur nutrition and environmental or nutritional stresses with regard to seed yield and protein composition; (ii) metabolic pathways that merit to be targeted to mitigate negative impacts of environmental stresses on seed protein quality; and (iii) the importance of sulfur homeostasis for the regulation of seed protein composition and its interplay with seed redox homeostasis.