All visitors from Vanuatu, Timor-Leste, Dominica, Honduras and Namibia now need a visa.
Britain's Home Secretary, Suella Braverman, said any decision to change a visa regime is not taken lightly.
"Careful consideration of Dominica's and Vanuatu's operation of a citizenship-by-investment scheme has shown clear and evident abuse of the scheme, including the granting of citizenship to individuals known to pose a risk to the UK," she said.
"The decision to impose these visa requirements has been taken solely for migration and border security reasons and is not a sign of poor relations with these countries.
"We keep our border and immigration system under regular review to ensure it continues to work in the UK national interest."
Braverman said arrangements are in place so the nationals of these countries can apply for visas.
There will be a four-week, visa-free transition period for those who hold confirmed bookings to the UK made on or before the new rules came in this week, as long as their arrival in the UK is no later than August 16.
"We have arrangements in hand to provide visas for diplomats from these countries currently working at the embassy in London," Braverman said.
The minister said nationals of these countries will also be required to obtain a Direct Airside Transit Visa if they intend to transit via the UK, having booked travel to another country.
The Vanuatu-based Daily Post reports Vanuatu had visa-free travel while the United Kingdom was part of the European Union.