Police said about 500 Pokomo fighters raided Kilelengwani village in Tana River Delta near dawn, setting fire to a police camp and other structures, Kenya's The Standard Digital reported.
The attackers allegedly engaged in a shootout with police that lasted several hours.
Red Cross officials said 12 men, five women, eight children and seven police officers are among the dead.
Witnesses said the attack was in apparent retaliation for a Friday attack by the Orma clan against the rival Pokomo tribe that left 11 people dead, Kenya's the Daily Nation reported.
Friday's attack was believed to be revenge for an August raid by the Pokomo tribe that left more than 50 people dead in a long-running dispute over grazing land and water, Voice of America reported.
More than 100 police officers had been deployed to the Tana River Delta to begin a government-backed program to disarm fighters in the area, The Standard Digital reported.