Species belonging to the Hydrangea genus, including Hydrangea macrophylla (Thunb.) Ser. and H. serrata (Thunb.) Ser., have inflorescences consisting of decorative and non-decorative flowers. The generation of double-flowered decorative flowers is an important objective for hydrangea breeders. This study aimed to elucidate the inheritance pattern of double flowers in H. macrophylla and H. serrata. Double-flowered progeny were obtained from three out of eleven crosses between double-flowered cultivars as the seed parent and single-flowered cultivars or lines as the pollen parent. When double-flowered progeny were produced using three cross combinations, the progenitor of both double-flowered cultivars and single-flowered lines was the double-flowered cultivar ‘Jogasaki’. In progeny obtained from these crosses, the segregation ratio of double- and single-flowered types was 1:1. Conversely, all progeny obtained from a cross in which the double-flowered cultivar as the seed parent differed from the seed parent of the single-flowered line as the pollen parent bore single flowers, and double-flowered progeny were not produced. Double-flowered progeny were obtained in two out of four crosses among the single-flowered lines; the segregation ratio of double- and single-flowered types was 1:3. The single-flowered lines shared the same seed parents, ‘Jogasaki’ or ‘Sumidanohanabi’. These results indicate that the double-flowered phenotype of decorative hydrangea flowers may be a recessive characteristic controlled by a single major gene. In addition, these results indicate that double-flowered progeny can be obtained when a pair of recessive genes are identical.