A tuffaceous claystone sample collected from a seamount flank of the Mariana Trench's ocean-ward slope by the Japanese submersible “Shinkai 6500” yielded very well-preserved earliest Cretaceous radiolarians. Initial spicule-bearing spherical radiolarians assignable to the families Centrocubidae and probably Entactiniidae have been identified in this radiolarian fauna. Based on the initial spicule and the connecting arches, we describe one new genus, Marianasphaera, belonging to the family Centrocubidae and another new genus called Shinkaiera, which is questionably assigned to the family Entactiniidae. Three new species, Marianasphaera ogawai, M. multispinosa, and Shinkaiera fragilis, are also described. These radiolarians provide important data, filling the gap between Triassic and Cenozoic initial spicule-bearing spherical radiolarians.