The seats will either be £19.99 or £29.99 for one-way travel during the peak summer months of June, July and August. Ryanair will support the promotion through national press ads and PR.
Chief executive Michael O'Leary, speaking at the firm's annual results announcement in London, said: "I can think of no better way to prove our determination to lower air fares than to announce a record low-fare seat sale."
Ryanair's net profits for the 12 months to March were £172m, two-and-a-half times higher than British Airways' for the same period.
O'Leary warned that last year's net profit margins of 28% were "non-sustainable" due to inevitably falling fares and declining load factors.
However, he did promise margins of about 20% for this year, while increasing traffic by a further 25%.
Feature, page 22.