Transport secretary Grant Shapps confirmed the additions to the banned list in a string of tweets on Thursday evening (21 January).
The new measures came into effect at 4am on Friday (22 January).
British and Irish nationals, as well as third-country nationals with residence rights, will be exempt.
"To help to stop the spread of the Covid-19 variant identified in South Africa, we are banning all arrivals from Tanzania and Democratic Republic of Congo from 4am tomorrow," said Shapps.
"All passengers from these countries except British and Irish nationals and third country nationals with residents rights will be denied entry.
"We are continuing to monitor Covid-19 rates and new strains of the virus across the globe. This, alongside the suspension of travel corridors and pre-departure testing, will help protect our borders."
Exempt arrivals from countries covered by the travel ban will be required to self-isolate for 10 days upon arrival, and will not be able to use the test to release scheme to shorten their quarantine.
Tanzania and the Democratic Republic of Congo bring the total number of countries and territories on the banned list to 30.
A ban on arrivals from southern Africa was announced shortly before Christmas owing to a new strain of coronavirus first identified in South Africa.
Most of South America was added in the new year after another new virulent strain was identified in Brazil.
Travel to the UK from Portugal has been banned due to the country’s historic travel links with Brazil, and Cape Verde in Africa owing to its ties with Portugal.
