This cross-sectional and descriptive study examined the family quality of life (FQoL) among 72 Brazilian families who have children with Williams syndrome, a rare genetic disorder in which most individuals have an intellectual disability, usually mild. Data were collected using sociodemographic and clinical data forms and the Beach Center FQoL Scale. The overall FQoL score was 3.90 ± 0.45, below the limit of four points considered satisfactory. Families felt more satisfied with the family interaction (4.11 ± 0.57), parenting (4.07 ± 0.42), and disability-related support (3.94 ± 0.62) domains, and less satisfied with the family's emotional (3.49 ± 0.73) and physical/material well-being (3.73 ± 0.74) domains. Paternal education, children's cardiopathy and autonomy in activities of daily living explained 24.5% of the variance in the overall FQoL. Measures are necessary to improve the emotional and physical/material well-being of families to reduce the family burden. Monitoring the child's cardiac condition and promoting independence in activities of daily living are also the main procedures.