The flights will likely start from the summer 2017 flight season, which begins in March.
German daily Frankfurter Allgemeine Zeitung, citing industry sources, said Ryanair would fly to popular Spanish tourist destinations such as Mallorca.
Airport operator Fraport has been in talks with low-cost carriers to boost passenger numbers at Europe’s fourth largest hub, and chief executive Stefan Schulte told Reuters last month that Fraport could offer financial incentives for a limited time to encourage new budget routes.
Ryanair boss Michael O’Leary has previously said high fees and long turnaround times at Frankfurt, London Heathrow and Paris Charles de Gaulle, make it difficult for low-cost operators to fly from those hubs.
The news will provide a further headache for Fraport’s main customer Lufthansa, which has slowed growth at its main brand and has faced strikes while it tries to lower costs and expand its own budget unit Eurowings.
In August Fraport cut its forecast for passenger numbers at Frankfurt this year, after attacks in Europe put off long-haul travellers.
Only a few low-cost carriers, including Spain’s Vueling and Iceland’s Wow Air, offer flights from Frankfurt.
Ryanair has been expanding in Germany, taking advantage of retrenchment by Air Berlin, to add flights from Hamburg and Berlin, including on the Berlin-Cologne route popular with business travellers.