Travellers using Booking.com, Kayak, Priceline and Agoda will now have access to pair the brands' accommodation options with Ryanair flights to more than 200 destinations.
Booking becomes Ryanair's latest "approved OTA" partner, a list that also includes other OTA giants like Loveholidays, which is the UK's third largest Atol holder and largest OTA Atol holder, as well as On the Beach and Expedia.
Booking.com is the UK's sixth largest Atol holder, and third largest OTA Atol holder after Loveholidays and On the Beach.
Tui also has a flight partnership with Ryanair, which allows agents to package Ryanair flights, and customers to do so themselves via the Tui website and app.
Customers booking though Booking Holdings' brands will also gain access to a myRyanair account without having to complete customer verification with the airline, meaning they will receive any essential flight updates directly.
Ryanair chief marketing officer Dara Brady said the new partnership would offer customers booking or comparing prices via Booking's brands "full price transparency".
"This partnership shows how 'Approved OTAs' and Ryanair can work together to protect and benefit consumers, giving them more choice at lower fares," he added.
Todd Henrich, Booking Holdings senior vice-president corporate development, said: “In bringing together access to Ryanair’s low-fare flights and network of more than 235 destinations with the innovative, intuitive technology, ease and flexibility offered by our brands, this partnership builds on our commitment to provide a trusted, seamless booking experience to customers all over the world.”