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.
