Officials admitted there was no 'immediate need' for the doses in the UK because they had received so many jabs.
They added that the plan would not derail the booster drive, or the roll out to 12 to 15-year-olds.
The deal comes two weeks after the UK agreed to send four million Pfizer jabs to Australia.
Both South Korea and Australia have struggled with anaemic vaccine roll outs in the face of delays to deliveries and the failure to secure enough jabs early.
Neither has double-jabbed more than 50 per cent of its adults. For comparison, the UK has fully inoculated almost eight in ten over-16s — or 44million people.
Britain has ordered more than 500million doses of Covid vaccines, enough to double-jab the population four times over.
It is expecting to receive 100million Pfizer doses by the end of this year, according to supply agreements. Some 40million Britons have already received the jab.
Under the 'vaccine swap' announced today the UK will send a million Pfizer doses to South Korea, with the first shipment set to leave in the coming weeks.
The Asian nation will then return the same number of Pfizer doses later this year.
The deal mirrors that struck with Australia, which has agreed to send four million Pfizer jabs to the UK by the end of this year.
Millions of Britons could be offered a third Covid vaccine in September.
A decision from the Joint Committee on Vaccination and Immunisation's (JCVI) about who should get a third dose is due by next month.
In interim guidance published in June, it set out framework for who should be considered:
Who could be offered a third Covid vaccine?All over-50s Frontline health and social care workers People who are vulnerable to the virus Adults living with vulnerable people