The Irish carrier blamed an 18% fall in the value of sterling since the Brexit vote as it cut its annual earnings forecast by 5%.
It said fares fell more than expected in the first half of its financial year - down 10% - and were now set to drop by up to 15% in the final six months as the Brexit-hit pound takes its toll.
Ryanair now expects full-year earnings of between 1.30 billion euros (£1.17 billion) and 1.35 billion euros (£1.22 billion), down from the previous range of 1.38 billion euros (£1.24 billion) to 1.43 billion euros (£1.28 billion).
It comes after rival easyJet revealed a £90 million impact from the falling pound earlier this month, on top of at least £125 million in lost profit after a combination of terror attacks across Europe, Egypt and Tunisia as well as air traffic control strikes in France and political turmoil in Turkey.
Ryanair and easyJet have also been slashing fares to boost demand.