BackgroundWe refine the clinical spectrum of FOXG1 syndrome and expand genotype–phenotype correlations through evaluation of 122 individuals enrolled in an international patient registry.MethodsThe FOXG1 syndrome online patient registry allows for remote collection of caregiver-reported outcomes. Inclusion required documentation of a (likely) pathogenic variant inFOXG1. Caregivers were administered a questionnaire to evaluate clinical severity of core features of FOXG1 syndrome. Genotype–phenotype correlations were determined using nonparametric analyses.ResultsWe studied 122 registry participants with FOXG1 syndrome, aged < 12 months to 24 years. Caregivers described delayed or absent developmental milestone attainment, seizures (61%), and movement disorders (58%). Participants harbouring a missense variant had a milder phenotype. Compared to individuals with gene deletions (0%) or nonsense variants (20%), missense variants were associated with more frequent attainment of sitting (73%). Further, individuals with missense variants (41%) achieved independent walking more frequently than those with gene deletions (0%) or frameshift variants (6%). Presence of epilepsy also varied by genotype and was significantly more common in those with gene deletions (81%) compared to missense variants (47%). Individuals with gene deletions were more likely to have higher seizure burden than other genotypes with 53% reporting daily seizures, even at best control. We also observed that truncations preserving the forkhead DNA binding domain were associated with better developmental outcomes.ConclusionWe refine the phenotypic spectrum of neurodevelopmental features associated with FOXG1 syndrome. We strengthen genotype-driven outcomes, where missense variants are associated with a milder clinical course.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13023-023-02745-y.