Mutations in SP7 (encoding osterix) have been identified as a rare cause of recessive osteogenesis imperfecta (‘OI type XII’) and in one case of dominant juvenile Paget's disease. We present the first description of young adult siblings with OI due to a unique heterozygous mutation in SP7. The phenotype was characterized by fragility fractures (primarily of the long bone diaphyses), poor healing, scoliosis, and dental malocclusion. Both siblings had very low cortical volumetric bone mineral density on peripheral quantitative computed tomography of the radius (z-scores −6.6 and − 6.7 at the diaphysis), porous cortices, and thin cortices at the radial metaphysis. Histomorphometry demonstrated thin cortices and low bone turnover with reduced osteoblast function. Both siblings were heterozygous for a missense variant affecting a highly conserved zinc finger domain of osterix (c.1019A > C;​ p.Glu340Ala) on DNA sequencing. Co-transfection of plasmids carrying the SP7 mutation with DLX5 and a luciferase reporter demonstrated that this variant impacted gene function (reduced transcription co-activation compared to wild-type SP7). The low cortical density and cortical porosity seen in our patients are consistent with previous reports of individuals with SP7 mutations. However, the low bone turnover in our patients contrasts with the high turnover state seen in previously reported patients with SP7 mutations. This report indicates that dominant variants in SP7 can give rise to OI. The predominant feature, low cortical density, is common in patients with other SP7 mutations, however other features appear to depend on the specific variant.