The drugs were sized while being transported in a boat in the Indian Ocean.
Tanzania's anti-narcotics watchdog said on Saturday it has seized more than one tonne of heroin and arrested seven suspects.
Gerald Kusaya, the commissioner-general of the Drug Control and Enforcement Authority (DCEA), said the drugs were seized in the early hours of Saturday while they were being transported in a boat in the Indian Ocean.
Anti-narcotics police in collaboration with the navy intercepted the heroin in the coastal waters off port town Kilwa Masoko in Lindi region, Kusaya told a news conference in the commercial capital Dar es Salaam.
Kusaya said all the seven suspects were found in the boat with the contraband.