Skeletal homeostasis critically depends on the proper anabolic functioning of osteolineage cells. Proliferation and matrix synthesis are highly demanding in terms of biosynthesis and bioenergetics, but the nutritional requirements that support these processes in bone-forming cells are not fully understood. Here, we show that glutamine metabolism is a major determinant of osteoprogenitor function during bone mass accrual. Genetic inactivation of the rate-limiting enzyme glutaminase 1 (GLS1) results in decreased postnatal bone mass, caused by impaired biosynthesis and cell survival. Mechanistically, we uncovered that GLS1-mediated glutamine catabolism supports nucleotide and amino acid synthesis, required for proliferation and matrix production. In addition, glutamine-derived glutathione prevents accumulation of reactive oxygen species and thereby safeguards cell viability. The pro-anabolic role of glutamine metabolism was further underscored in a model of parathyroid hormone (PTH)-induced bone formation. PTH administration increases glutamine uptake and catabolism, and GLS1 deletion fully blunts the PTH-induced osteoanabolic response. Taken together, our findings indicate that glutamine metabolism in osteoprogenitors is indispensable for bone formation. © 2020 American Society for Bone and Mineral Research (ASBMR).