At least 45 people were wounded in the simultaneous attacks in Garissa, in the north of the East African country, which has suffered a series of attacks since sending troops into Somalia last October to crush its northern neighbour's al Shabaab militants.
There was no immediate claim of responsibility, but police believe the attacks could have been the work of al Shabaab sympathisers.
Regional deputy police chief Philip Ndolo said: "They were clad in balaclavas."
Seven attackers threw grenades inside the Catholic Church and the African Inland Church, two miles apart, yesterday morning. They then opened fire with guns.
Two policemen were among the dead.
The attacks were the latest on Christians in Kenya after two worshippers were killed in grenade blasts in March and April.
The attacks on churches resembled the tactics of Nigeria's Islamist militant group Boko Haram, which has killed hundreds of people on the other side of the continent.
Other blasts in Kenya have hit nightclubs and bus stations in the capital Nairobi, the coastal city of Mombasa, and areas near the Somalia border.