Background Isolated synostosis of the frontosphenoidal suture is very rare and difficult to diagnose. Little has been reported on the clinical presentation and fetal development of this suture. Objective To understand the development of the frontosphenoidal suture and the outcome of its synostosis. Materials and methods We studied the normal fetal development of the frontosphenoidal suture in dry human skulls and the clinical features of four patients with isolated synostosis of the frontosphenoidal suture. Results The frontosphenoidal suture develops relatively late during the second trimester of pregnancy, which explains the mild phenotype when there is synostosis. This rare craniosynostosis results in a deformity that causes recession of the lateral part of the frontal bone and supraorbital rim, with minimal facial asymmetry. Three-dimensional CT is the best examination to confirm the diagnosis. Conclusion Isolated frontosphenoidal synostosis should be considered in patients with unilateral flattening of the forehead at birth that does not improve within the first few months of life.