The disease broke out in Brazzaville but has also affected parts of neighboring Democratic Republic of Congo and Angola, the BBC reported.
The Congo-Brazzaville government said the vast majority of deaths had occurred in the city of Pointe-Noire.
The vaccination plan is being conducted by several aid agencies, including UNICEF and the World Health Organization.
"The first round of a mass vaccination campaign targeting 3 million people will begin Friday, in response to a polio epidemic which has unusually claimed a majority of adult victims," a joint statement from the agencies involved said.
Polio normally strikes young children, officials said.
The immunization plan will start in Pointe-Noire, the epicenter of the outbreak, and expand into surrounding areas.
"Every man, every woman, every child will be immunized irrespective of their past immunization status," Dr. Luis G. Sambo, WHO's regional director for Africa, said.
Congo's director general of health, Alexis Elira Dokekias, said the adult victims had either not been sufficiently immunized or not immunized at all.