Chief executive Michael O’Leary said it will return to the eastern European country “when it’s safe to do so” but warned that might not be until the winter.
The airline was previously the largest to serve Ukraine.
Ukrainian airspace was closed last week following Russia’s invasion.
Speaking at a press conference in the City of London, Mr O’Leary said: “We do want to see the Ukrainians succeed.”
He added that the best way to punish Russia would be for the West to “drive down the price of oil”.
“We’ve cancelled all Ukrainian routes until the end of March,” he said.
“We do not believe it will be possible to fly to Ukraine for the foreseeable future.”
The “first thing” the Russians did when they invaded was “disable the flight systems” at major airports, he added.
He went on: “We will be the first airline to return to Ukraine when it’s safe to do so.
“But I suspect it will take probably maybe into next winter before those technologies can be restored at Ukrainian airports, when hopefully the Ukrainians will have seen off the Russians and sent them back to where they came from.”
Mr O’Leary said bookings across all routes on Thursday and Friday last week were down by “about 20%” compared with a week earlier.
Over the weekend and on Monday they were “down about 10%”.
He added: “I think it will not have a dramatic impact on bookings as long as the war doesn’t escalate and spread elsewhere.”
The airline has seen a “significant surge” in bookings to and from Poland, as people fleeing Ukraine are “reuniting with family and friends” across Europe.
Mr O’Leary noted that many people fleeting Ukraine have relatives “dotted across Europe”.
Ryanair will carry an estimated 44 million passengers on UK flights this year.
Mr O’Leary predicted that this will lead to it overtaking easyJet to become the UK’s largest airline.
Ryanair is operating its largest summer schedule from its three London airports, Gatwick, Luton and Stansted.
It is introducing 14 new routes from the airports to destinations such as Naples, Madeira and Stockholm.