Gunmen shot dead a Catholic priest outside his church in the capital of Zanzibar on Sunday, the second such attack on the Muslim-majority island in recent months.
"Father Evarist Mushi was blocked by two young men at the entrance of the church, one of the attackers shot him in the head," said the island's police spokesman Mohammed Mhina.
On Christmas Day, gunmen shot and seriously wounded a Catholic priest as he returned home from church.
It is not clear if the two attacks were linked, or religiously motivated.
In November, attackers threw acid in the face and chest of a Muslim cleric, and there have been tensions between the two communities in the semi-autonomous Tanzanian archipelago.
"For now, we cannot say what the motives are for the attack, but we have launched search operations for the attackers," added Mhina.
Zanzibar's Christian community is a minority there, an estimated three percent of the 1.2 million population, which is otherwise almost entirely Muslim.
Tourism is the main foreign currency earner for Zanzibar, famed for its white-sand beaches and historical buildings in Stone Town, listed as a world heritage site by UNESCO.