Deliveries are expected to start by the end of this year, assuming the vaccine is cleared by regulators and approved for general use.
The deal comes after Pfizer and BioNTech announced on Monday that their experimental COVID-19 vaccine was more than 90 percent effective based on initial trial results.
"In the wake of Monday's promising announcement by BioNTech and Pfizer on the prospects for their vaccine, I'm very happy to announce today's agreement ... to purchase 300 million doses," European Commission President Ursula von der Leyen said in a statement on Wednesday.
European Health Commissioner Stella Kyriakides was quoted as saying that the vaccine deal followed "the encouraging first indications from the clinical trial results" and was "a very telling example of what the EU can achieve when working together, as a Union."
Kyriakides tweeted on Thursday that "a safe and effective COVID-19 vaccine will not return us to normality overnight, but will play a major role in saving lives and livelihoods, restoring our economy and protecting our healthcare systems."
Poland’s Prime Minister Mateusz Morawiecki said on Monday that millions of vaccines against the coronavirus were likely to reach his country next spring as part of a European deal with drug makers.
He added that the vaccine would be made available to citizens on an optional basis and be first administered to senior citizens, healthcare professionals and law enforcement personnel.
Morawiecki also said that he had issued instructions for an infrastructure to be put in place next year for nationwide vaccinations against COVID-19.
The Polish prime minister said in a social media post on Thursday that Poles would be able to access the vaccine "as soon as it's ready."
Poland on Thursday reported 22,683 new coronavirus infections and 275 more deaths, bringing its total number of cases to 641,496 and fatalities to 9,080.
Source: PAP, ec.europa.eu
Click on the audio player above for a report by Radio Poland's Sławek Szefs.