Financial terms of the deal were not disclosed.
No vaccine has yet been approved to treat or prevent COVID-19.
The UK could start immunizing priority groups, such as frontline social and health workers and those at increased risk of coronavirus, if treatment proves effective.
The vaccinations will take place as early as the first half of next year, the Department of Business, Energy and Industrial Strategy (BEIS) said.
Human clinical studies of the vaccine will begin in September, followed by a phase 3 study in December.
The government has now signed deals for four different types of potential coronavirus vaccines and a total of 250 million doses.
Business Secretary Alok Sharma said, “Our scientists and researchers are rushing to find a safe and effective vaccine at a speed and scale never seen before.
“If this progress is truly remarkable, the fact remains that there is no guarantee.
“In the meantime, it’s important that we gain early access to a wide range of promising vaccine candidates, like GSK and Sanofi, to increase our chances of finding one that works so we can protect the public and save lives.
Sanofi and GSK, which first partnered in April, confirmed in a statement that regulatory approval for their vaccine could be obtained by the first half of 2021 if clinical data were positive.