The low-cost carrier has implemented the change after it a successful trial in April.
All passengers will be allocated a seat for free but can pay extra for more leg room (GBP12), front seats (GBP8) or GBP3 for other seats.
easyJet plus! Cardholders and Flexi Fare customers can select a seat free of charge and still check in and board before others.
“Today’s move offering allocated seating on all of our flights is the single, biggest change the airline has undertaken in its history and an example of easyJet trying to do all it can to make travel easy and affordable for our passengers,” said Paul Simmons, UK director at easyJet.