From next month, those passengers who do not pay to reserve a seat will only be able to check in from 48 hours before departure.
The low-cost carrier hopes the change will encourage fliers to pay to select a seat.
If customers pay for a reserved seat – which costs from £3 each way but rises to £15 for seats with more leg room – they will be allowed to check in up to 60 days before flying.
Ryanair pointed out that, though the 48-hour window is down from four days, it is still double the 24 hour check-in period operated by Aer Lingus, British Airways, Lufthansa, Norwegian and Iberia.
“This will give reserve seat customers more time to select their preferred seats prior to departure,” the Irish carrier said.
As recently as 2016, passengers could check in for free up to a week before departure.