Euro 2016 spread across 11 venues
The decision means that Toulouse and St Etienne, reserve cities until now, will join the list of fully-fledged hosts.
Platini said the request from the French organisers made sense as 24 teams will take part in the tournament rather than the present 16.
"With 20-odd extra matches to play, it makes perfect sense and it will allow us to distribute revenue of the competition in the fairest manner," Platini told reporters.
"It will be good for them to be included and will help the improve the infrastructure of football throughout France.
"I believe the decision of the executive committee was extremely good, it represents common sense and what's best for football, hosts cities and the organising country."
Platini said very high standards would be expected from host cities.
"We know some cities may have some trouble and we may come down from 11 cities to nine," he said.
"But that's better than coming down from nine to seven."
Matches will also be played at the Stade de France and Parc des Princes in Paris, Nancy, Lille, Lyon, Marseille, Lens, Bordeaux and Nice.